Stringify correctly for non-str exception argument

Stringify correctly for non-str exception argument

Fixed issue where a non-string object sent to
:class:`_exc.SQLAlchemyError` or a subclass, as occurs with some third
party dialects, would fail to stringify correctly. Pull request
courtesy Andrzej Bartosiński.

Fixes: #5599
Closes: #5600
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5600
Pull-request-sha: cdccccc42a6ac8de771593a43ee8675bfd8dbeb6

Change-Id: Icd710d9015abc80f61a84893d75fbb33ee0fe46e
(cherry picked from commit c4f28fb414c342ba362fdee8a68bd169f01aa7d2)