Leave bytestring exception messages as bytestrings

Leave bytestring exception messages as bytestrings

Fixed a regression introduced in version 1.2 where a refactor
of the :class:`.SQLAlchemyError` base exception class introduced an
inappropriate coercion of a plain string message into Unicode under
python 2k, which is not handled by the Python interpreter for characters
outside of the platform's encoding (typically ascii).  The
:class:`.SQLAlchemyError` class now passes a bytestring through under
Py2K for ``__str__()`` as is the behavior of exception objects in general
under Py2K, does a safe coercion to unicode utf-8 with
backslash fallback for ``__unicode__()``.  For Py3K the message is
typically unicode already, but if not is again safe-coerced with utf-8
with backslash fallback for the ``__str__()`` method.

Fixes: #4429
Change-Id: I2289da3f2c45c7d0041fa43d838958f7614defc3