Fixed #15889 -- when trying to access to access a serializer that doesn't exist, raise a new SerializerDoesNotExist exception.  Thanks to Mathieu Agopian for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16104 bcc190cf-cafb-0310-a4f2-bffc1f526a37