[1.2.X] Fixed #13190 -- Improved error handling for the case where no...

[1.2.X] 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.

Backport of r14793 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14799 bcc190cf-cafb-0310-a4f2-bffc1f526a37