Improved the auth admin site to raise Http404 with a helpful error message if...

Improved the auth admin site to raise Http404 with a helpful error message if DEBUG is True, explaining why permission isn't denied. Refs #9866, and see also [9682]

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9683 bcc190cf-cafb-0310-a4f2-bffc1f526a37