[1.0.X] Fixed #10118 -- Clarified the error message raised when accessing a...

[1.0.X] Fixed #10118 -- Clarified the error message raised when accessing a subclass model that doesn't exist. Thanks to peterbraden@peterbraden.co.uk for the suggestion.

Merge of r9859 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9869 bcc190cf-cafb-0310-a4f2-bffc1f526a37