Fixed #12004 -- Improved error reporting when an abstract class is registered...

Fixed #12004 -- Improved error reporting when an abstract class is registered with the admin. Thanks to Matt Smalley for the report, and to mk and Julien Phalip for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15636 bcc190cf-cafb-0310-a4f2-bffc1f526a37