exception Bad_format now takes a backtrace list to preserve through re-raising

Also turns on debug, backtraces are only dumped when Bad_format is re-raised
Backtraces are only printed if backtraces are turned on
Use formatted strings for backtraces rather than raw_backtraces
Rewriting description of option -no-aspcud, so that it does not mention aspcud.
Still keeping the name -no-aspcud, though, for backwards compatibility
Added alias --use-internal-solver to --no-aspcud, which is deprecated.
Added environment variable OPAMUSEINTERNALSOLVER corresponding to the option --use-internal-solver
Merge pull request #1277 from rdicosmo/add-solver-option

Add --use-internal-solver option, keep --no-aspcud as an alias
Merge pull request #1276 from dsheets/format-error-backtraces

exception Bad_format now takes a backtrace list to preserve through re-raising
Fail rather than do nonsense when external solver returns a bogus answer

OpamClient.preprocess_request has not been performed in that case, and that can
lead the internal heuristic to remove every package unrelated to the request.
It would sure be better to retry cleanly, but for now this is strictly better, and
anyway this shouldn't happen.
Merge pull request #1279 from OCamlPro/no-inconsistent-solver-nonsense

Fail rather than do nonsense when external solver returns a bogus answer
Cleanup some duplicate error messages, center message headers
Better error messages on 'opam pin' command-line option

Closes #1278. Thanks for reporting !
Fix bug with 'opam show' on locally-pinned packages
Better error message on requests with pinned packages
Store real versions with `switch export` and pinned packages

Partially fixes #1265, but we need to fix that Not_found as well. Would you have a backtrace ?

This should prevent failures, but wont re-import the pinned state of packages. We should add
it without breaking the existing format...
Cleaned up some messages
Fixes to Properly handle re-pinning, pin editing, etc. and reinstallations
Merge pull request #1283 from OCamlPro/pinning-goodness

Fixes a bunch of issues, mainly related to live (re-)pinning of packages
Make get-git-id work event in detached HEAD.
Explain changes in comments.
Merge pull request #1284 from rdicosmo/fix-get-git-version

Fix get git version