Fixed #7198 (again) -- Corrects a problem with string interpolation from...

Fixed #7198 (again) -- Corrects a problem with string interpolation from r16876 and adds tests for the new error message.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@16888 bcc190cf-cafb-0310-a4f2-bffc1f526a37