Fixed #7241 -- More robust exception catching in the transaction management

code. As pointed out in the ticket, Python still lets you raise all sorts of
odd things as exceptions (e.g. strings), so even though they're bad form, we
should still handle them. We do that cleanly now. Thanks to jim-django@dsdd.org
for the patch.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8419 bcc190cf-cafb-0310-a4f2-bffc1f526a37