Fixed #11957 -- exceptions in admin.py are no longer hidden after second request

Before you had to restart runserver for the correct exception message to show
up again. Reverts fix in r9680 which has this side-affect.

Thanks to jarrow, carljm and ramiro for their work on the patch and tickets.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12956 bcc190cf-cafb-0310-a4f2-bffc1f526a37