Fixed #12237 -- Improved the error message for m2m fields with an explicit...

Fixed #12237 -- Improved the error message for m2m fields with an explicit through model being listed in admin fieldsets. Thanks to Pyth for the report and Ramiro Morales for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11744 bcc190cf-cafb-0310-a4f2-bffc1f526a37