Fixed #7471 -- If the 400 response handler raises an exception, pass control to

the 500 handler (if that then raises an exception, it's just not your day).

Patch from Leah Culver.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7988 bcc190cf-cafb-0310-a4f2-bffc1f526a37