[1.2.X] Fixed #15532 -- Clarified the error message for unknown auth backends....

[1.2.X] Fixed #15532 -- Clarified the error message for unknown auth backends. Thanks to kmike for the report.

Backport of r15713 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15715 bcc190cf-cafb-0310-a4f2-bffc1f526a37