Fixed #5531 -- Changes a while back meant we are handling import errors from

database backends differently now. Which meant the MySQLdb version check was
being swallowed. Changed the exception type to ensure this is percolated
correctly. Patch from Ramiro Morales.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7358 bcc190cf-cafb-0310-a4f2-bffc1f526a37