[1.1.X] Fixed #12152. DoesNotExist and MultipleObjectsReturned now subclass...

[1.1.X] Fixed #12152. DoesNotExist and MultipleObjectsReturned now subclass their parent model's exceptions. Backport of r12567 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12568 bcc190cf-cafb-0310-a4f2-bffc1f526a37