The main change is to treat error_already_set as a separate category
of exception that arises in different circumstances and needs to be
handled differently. The asymmetry between Python and C++ exceptions
is further emphasized.
To deal with exceptions that hit destructors or other noexcept functions.
Includes fixes to support Python 2.7 and extends documentation on
error handling.
@virtuald and @YannickJadoul both contributed to this PR.
