Fixed #13190 -- Improved error handling for the case where no authentication...

Fixed #13190 -- Improved error handling for the case where no authentication backends are defined. Thanks to Joel3000 for the report, and Łukasz Rekucki for the final patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14793 bcc190cf-cafb-0310-a4f2-bffc1f526a37