Summary:
Update hg_import_helper.py to include the exception type name in error
responses. Add a new HgImportPyError class in the C++ code to include both the
python exception type name and the message string.
In the future this will give us the ability to perform special handling based
on the python exception type, rather than just on the message contents.
Reviewed By: bolinfest
Differential Revision: D6333613
fbshipit-source-id: 1074bbf9fa25ee8b1abeadc38b1a4f569bc18d13
