Give a better error message on a tactic
Port Exponential Law proofs from HoTT-categories

Unfortunately, they're pretty slow.

After     | File Name                                                | Before
-------------------------------------------------------------------------------
16m32.28s | Total                                                    | 8m18.49s
-------------------------------------------------------------------------------
2m52.61s  | categories/ExponentialLaws/Law2/Law                      |   N/A
2m31.12s  | categories/ExponentialLaws/Law4/Law                      |   N/A
2m28.86s  | categories/ExponentialLaws/Law3/Law                      |   N/A
1m51.61s  | categories/Comma/ProjectionFunctors                      | 1m46.99s
0m35.75s  | categories/Adjoint/Composition/AssociativityLaw          | 0m36.01s
0m26.41s  | categories/Grothendieck/PseudofunctorToCat               | 0m24.69s
0m18.30s  | categories/Adjoint/UnitCounitCoercions                   | 0m16.62s
0m17.32s  | categories/ExponentialLaws/Law4/Functors                 | 0m17.31s
0m16.88s  | categories/Comma/InducedFunctors                         | 0m15.46s
0m16.11s  | categories/FunctorCategory/Dual                          | 0m15.73s
0m14.50s  | categories/Adjoint/Dual                                  | 0m12.01s
0m13.80s  | categories/Adjoint/Composition/Core                      | 0m12.06s
0m13.74s  | hit/Flattening                                           | 0m12.68s
0m12.61s  | categories/Pseudofunctor/Core                            | 0m12.44s
0m12.60s  | categories/Functor/Pointwise/Properties                  | 0m12.53s
0m11.96s  | categories/Adjoint/Composition/IdentityLaws              | 0m10.77s
0m10.67s  | categories/Pseudofunctor/FromFunctor                     | 0m10.43s
0m08.58s  | categories/ExponentialLaws/Law1/Law                      | 0m08.14s
0m08.21s  | categories/Functor/Prod/Universal                        | 0m08.24s
0m08.12s  | categories/GroupoidCategory/Morphisms                    | 0m08.06s
0m07.70s  | categories/Adjoint/Paths                                 | 0m06.51s
0m07.19s  | hit/Connectedness                                        | 0m07.08s
0m06.94s  | categories/UniversalProperties                           | 0m06.70s
0m06.38s  | categories/Functor/Paths                                 | 0m05.97s
0m06.31s  | categories/Adjoint/UnitCounit                            | 0m05.69s
0m06.17s  | categories/Comma/Dual                                    | 0m06.21s
0m06.10s  | categories/Category/Morphisms                            | 0m05.85s
0m05.81s  | categories/Functor/Sum                                   | 0m05.79s
0m05.53s  | categories/DualFunctor                                   | 0m05.61s
0m05.30s  | EquivalenceVarieties                                     | 0m05.08s
0m05.22s  | categories/ExponentialLaws/Law2/Functors                 | 0m04.80s
0m04.05s  | categories/Category/Sigma/OnMorphisms                    | 0m04.17s
0m03.68s  | categories/ProductLaws                                   | 0m03.68s
0m03.55s  | types/ObjectClassifier                                   | 0m03.62s
0m03.10s  | categories/Adjoint/UniversalMorphisms                    | 0m02.86s
0m02.98s  | types/Sigma                                              | 0m03.00s
0m02.71s  | categories/Functor/Prod/Functorial                       | 0m02.77s
0m02.68s  | Equivalences                                             | 0m02.60s
0m02.64s  | categories/Grothendieck/ToSet                            | 0m02.78s
0m02.60s  | categories/FunctorCategory/Functorial                    | 0m02.56s
0m02.28s  | categories/Functor/Attributes                            | 0m02.36s
0m02.26s  | categories/Category/Sum                                  | 0m02.21s
0m02.23s  | categories/Comma/Core                                    | 0m02.09s
0m02.16s  | PathGroupoids                                            | 0m01.95s
0m01.99s  | categories/Category/Sigma/Core                           | 0m02.12s
0m01.87s  | categories/NaturalTransformation/Dual                    | 0m01.80s
0m01.79s  | categories/NaturalTransformation/Isomorphisms            | 0m01.88s
0m01.75s  | categories/HomFunctor                                    | 0m01.80s
0m01.64s  | categories/NaturalTransformation/Paths                   | 0m01.67s
0m01.59s  | categories/FunctorCategory/Morphisms                     | 0m01.72s
0m01.39s  | categories/Category/Sigma/OnObjects                      | 0m01.50s
0m01.35s  | categories/Limits/Core                                   | 0m01.17s
0m01.30s  | categories/NaturalTransformation/Pointwise               | 0m01.30s
0m01.28s  | types/Prod                                               | 0m01.30s
0m01.26s  | categories/Category/Prod                                 | 0m01.36s
0m01.18s  | categories/NaturalTransformation/Composition/Laws        | 0m01.13s
0m01.14s  | categories/Functor/Pointwise                             | 0m01.12s
0m01.04s  | categories/CategoryOfSections/Core                       | 0m01.05s
0m01.03s  | categories/ExponentialLaws/Law1/Functors                 | 0m01.02s
0m01.02s  | categories/NaturalTransformation/Prod                    | 0m01.04s
0m01.00s  | hit/Spheres                                              | 0m00.96s
0m00.99s  | categories/ExponentialLaws/Law   N/A                     | 0m01.01s
0m00.94s  | categories/Functor/Prod                                  | 0m00.88s
0m00.85s  | types/Paths                                              | 0m00.86s
0m00.84s  | hit/epi                                                  | 0m00.83s
0m00.81s  | Trunc                                                    | 0m00.84s
0m00.80s  | categories/Functor/Composition/Functorial                | 0m00.82s
0m00.65s  | categories/InitialTerminalCategory                       | 0m00.67s
0m00.64s  | types/Universe                                           | 0m00.62s
0m00.64s  | categories/NaturalTransformation/Composition/Core        | 0m00.68s
0m00.61s  | Functorish                                               | 0m00.56s
0m00.59s  | categories/categories                                    | 0m00.53s
0m00.58s  | types/Sum                                                | 0m00.57s
0m00.57s  | categories/Cat/Core                                      | 0m00.38s
0m00.57s  | categories/NaturalTransformation/Sum                     | 0m00.60s
0m00.57s  | hit/quotient                                             | 0m00.56s
0m00.56s  | categories/ExponentialLaws/Law3/Functors                 | 0m00.46s
0m00.51s  | categories/Comma/Projection                              | 0m00.52s
0m00.49s  | types/Record                                             | 0m00.48s
0m00.48s  | hit/Suspension                                           | 0m00.48s
0m00.47s  | hit/Circle                                               | 0m00.51s
0m00.46s  | Tactics                                                  | 0m00.42s
0m00.46s  | categories/Functor/Composition/Laws                      | 0m00.48s
0m00.46s  | types/Forall                                             | 0m00.42s
0m00.45s  | categories/Limits/Functors                               | 0m00.48s
0m00.44s  | categories/NaturalTransformation/Identity                | 0m00.37s
0m00.42s  | Fibrations                                               | 0m00.35s
0m00.42s  | categories/KanExtensions/Functors                        | 0m00.42s
0m00.40s  | TruncType                                                | 0m00.33s
0m00.40s  | HProp                                                    | 0m00.42s
0m00.39s  | HSet                                                     | 0m00.38s
0m00.39s  | categories/Functor/Functor                               | 0m00.38s
0m00.37s  | hit/minus1Trunc                                          | 0m00.25s
0m00.30s  | Misc                                                     | 0m00.32s
0m00.30s  | categories/NaturalTransformation/Composition/Functorial  | 0m00.27s
0m00.29s  | categories/ExponentialLaws/ExponentialLaws               | 0m00.32s
0m00.28s  | categories/Cat/Morphisms                                 | 0m00.28s
0m00.28s  | categories/Grothendieck/Grothendieck                     | 0m00.29s
0m00.28s  | categories/KanExtensions/Core                            | 0m00.38s
0m00.27s  | categories/IndiscreteCategory                            | 0m00.28s
0m00.27s  | categories/Functor/Dual                                  | 0m00.32s
0m00.26s  | categories/DependentProduct                              | 0m00.21s
0m00.26s  | categories/Functor/Composition/Core                      | 0m00.22s
0m00.26s  | types/Arrow                                              | 0m00.27s
0m00.25s  | categories/Category/Objects                              | 0m00.26s
0m00.24s  | categories/Notations                                     | 0m00.24s
0m00.23s  | categories/Adjoint/Adjoint                               | 0m00.22s
0m00.22s  | categories/NaturalTransformation/NaturalTransformation   | 0m00.18s
0m00.22s  | categories/Grothendieck/ToCat                            | 0m00.22s
0m00.22s  | categories/SetCategory/Functors/Functors                 | 0m00.17s
0m00.22s  | categories/Utf8                                          | 0m00.22s
0m00.20s  | categories/NatCategory                                   | 0m00.22s
0m00.19s  | categories/FunctorCategory/Utf8                          | 0m00.17s
0m00.19s  | hit/iso                                                  | 0m00.20s
0m00.18s  | Conjugation                                              | 0m00.21s
0m00.18s  | categories/NaturalTransformation/Core                    | 0m00.14s
0m00.18s  | categories/Pseudofunctor/Pseudofunctor                   | 0m00.15s
0m00.18s  | categories/NaturalTransformation/Notations               | 0m00.17s
0m00.18s  | FunextVarieties                                          | 0m00.14s
0m00.17s  | categories/Functor/Prod/Prod                             | 0m00.19s
0m00.17s  | categories/FunctorCategory/FunctorCategory               | 0m00.20s
0m00.17s  | categories/Functor/Notations                             | 0m00.14s
0m00.17s  | categories/FunctorCategory/Core                          | 0m00.16s
0m00.16s  | categories/Comma/Utf8                                    | 0m00.13s
0m00.16s  | categories/Adjoint/Core                                  | 0m00.13s
0m00.16s  | categories/Adjoint/Utf8                                  | 0m00.14s
0m00.16s  | categories/Limits/Limits                                 | 0m00.14s
0m00.16s  | categories/Cat/Cat                                       | 0m00.16s
0m00.16s  | categories/Comma/Comma                                   | 0m00.17s
0m00.16s  | categories/Profunctor/Profunctor                         | 0m00.12s
0m00.16s  | categories/GroupoidCategory/Core                         | 0m00.18s
0m00.16s  | categories/Comma/Notations                               | 0m00.12s
0m00.16s  | categories/Adjoint/Composition/Composition               | 0m00.17s
0m00.16s  | categories/Functor/Utf8                                  | 0m00.18s
0m00.16s  | categories/Profunctor/Representable                      | 0m00.17s
0m00.16s  | categories/Adjoint/Composition/LawsTactic                | 0m00.14s
0m00.16s  | categories/Profunctor/Utf8                               | 0m00.12s
0m00.15s  | categories/ExponentialLaws/Law3/Law3                     | 0m00.12s
0m00.15s  | HoTT                                                     | 0m00.08s
0m00.15s  | hit/unique_choice                                        | 0m00.13s
0m00.15s  | categories/Category/Notations                            | 0m00.11s
0m00.15s  | categories/ExponentialLaws/Law1/Law1                     | 0m00.16s
0m00.15s  | Overture                                                 | 0m00.15s
0m00.14s  | categories/Category/Dual                                 | 0m00.15s
0m00.14s  | categories/Adjoint/Identity                              | 0m00.15s
0m00.14s  | categories/Adjoint/Notations                             | 0m00.16s
0m00.14s  | categories/Category/Category                             | 0m00.18s
0m00.14s  | categories/NaturalTransformation/Utf8                    | 0m00.18s
0m00.13s  | categories/KanExtensions/KanExtensions                   | 0m00.14s
0m00.13s  | hit/Interval                                             | 0m00.11s
0m00.13s  | categories/SetCategory/Core                              | 0m00.14s
0m00.13s  | categories/Category/Univalent                            | 0m00.13s
0m00.13s  | categories/Category/Core                                 | 0m00.14s
0m00.13s  | categories/SetCategory/SetCategory                       | 0m00.09s
0m00.13s  | categories/NaturalTransformation/Composition/Composition | 0m00.13s
0m00.13s  | categories/Functor/Composition/Composition               | 0m00.12s
0m00.13s  | hit/Truncations                                          | 0m00.11s
0m00.12s  | categories/DiscreteCategory                              | 0m00.11s
0m00.12s  | categories/Category/Utf8                                 | 0m00.14s
0m00.12s  | types/Bool                                               | 0m00.13s
0m00.12s  | categories/ExponentialLaws/Law2/Law2                     | 0m00.13s
0m00.12s  | categories/Functor/Pointwise/Pointwise                   | 0m00.09s
0m00.12s  | categories/FunctorCategory/Notations                     | 0m00.10s
0m00.11s  | categories/GroupoidCategory/GroupoidCategory             | 0m00.10s
0m00.11s  | categories/Functor/Identity                              | 0m00.07s
0m00.11s  | coq/Init/Peano                                           | 0m00.08s
0m00.11s  | contrib/HoTTBook                                         | 0m00.16s
0m00.11s  | categories/Functor/Core                                  | 0m00.10s
0m00.11s  | categories/Profunctor/Identity                           | 0m00.13s
0m00.11s  | UnivalenceAxiom                                          | 0m00.12s
0m00.11s  | types/Unit                                               | 0m00.12s
0m00.11s  | categories/Category/Subcategory/Full                     | 0m00.10s
0m00.11s  | categories/CategoryOfSections/CategoryOfSections         | 0m00.12s
0m00.10s  | Contractible                                             | 0m00.13s
0m00.10s  | categories/Profunctor/Core                               | 0m00.12s
0m00.10s  | categories/Profunctor/Notations                          | 0m00.12s
0m00.10s  | categories/ExponentialLaws/Law4/Law4                     | 0m00.11s
0m00.10s  | categories/Category/Subcategory/Wide                     | 0m00.11s
0m00.10s  | categories/Category/Subcategory/Subcategory              | 0m00.10s
0m00.10s  | Pointed                                                  | 0m00.10s
0m00.10s  | categories/Category/Sigma/Sigma                          | 0m00.12s
0m00.09s  | types/Empty                                              | 0m00.08s
0m00.08s  | coq/Init/Datatypes                                       | 0m00.05s
0m00.08s  | categories/Category/Strict                               | 0m00.08s
0m00.08s  | coq/Init/Specif                                          | 0m00.09s
0m00.06s  | coq/Init/Logic_Type                                      | 0m00.06s
0m00.06s  | coq/Program/Tactics                                      | 0m00.04s
0m00.05s  | coq/Init/Notations                                       | 0m00.04s
0m00.05s  | coq/Init/Prelude                                         | 0m00.04s
0m00.05s  | FunextAxiom                                              | 0m00.07s
0m00.04s  | coq/Init/Tactics                                         | 0m00.04s
0m00.02s  | coq/Init/Logic                                           | 0m00.03s
0m00.02s  | coq/Bool/Bool                                            | 0m00.04s