Fixed #7026 -- Fixed misleading/incorrect exception text when adding to a...

Fixed #7026 -- Fixed misleading/incorrect exception text when adding to a many-to-many set on an object that doesn't yet have a primary-key value. Thanks for the report, ryan@peaceworks.ca

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7622 bcc190cf-cafb-0310-a4f2-bffc1f526a37