Fixed #10118 -- Clarified the error message raised when accessing a subclass...

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.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9859 bcc190cf-cafb-0310-a4f2-bffc1f526a37