ENH:  GH2578, allow ix and friends to partially set when the key is not contained
      in the object
API: iat/iloc will raise if enlargement is specified as its ambiguous

CLN: set_value in Series/Frame now go thru indexing routings in core/indexing.py
Merge pull request #4515 from jreback/partial_setting

ENH/API:  GH2578, allow ix and friends to partially set when the key is not contained in the object
ENH:Add raise_with_traceback & use in assertRaises

* Adds `raise_with_traceback` to `pandas.compat`, which handles the
  Python 2/3 syntax differences for raising with traceback.
* Uses `raise_with_traceback` in `assertRaises` and `assertRaisesRegexp`
  so they provide better feedback.

ENH: raise_with_traceback method.
CLN: Better exceptions and tests for core/frame

CLN: Big simplification of test_frame
TST: Add test cases for bad input, etc.
CLN: Exceptions that shouldn't happen should be assertions (mostly _apply*)
CLN/TST: Right Exceptions + remove duplicate tests

A few tests were duplicating the same thing or actually just the same
test, those have been refactored or removed. Many tests were changed to
specify the Exception they were looking for (and also to use the new
`with_statement` format for assertRaises, etc.)

CLN: Harmonize SparseDF and DF Exceptions
ENH: SparseDF apply now accepts axis='columns'