Fixed #8957 -- Fixed incorrect error message when Admin prepopulated_fields...

Fixed #8957 -- Fixed incorrect error message when Admin prepopulated_fields refers to a field that does not exist. Thanks, charmless

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9002 bcc190cf-cafb-0310-a4f2-bffc1f526a37