Refs #12203 -- Improved error handling for the case where a user manually...

Refs #12203 -- Improved error handling for the case where a user manually specifies an m2m field with an explicit through field. Thanks to dgouldin for the report, and Ramiro Morales for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11737 bcc190cf-cafb-0310-a4f2-bffc1f526a37