Changed all model unit tests to use __str__() instead of __repr__(). Also...

Changed all model unit tests to use __str__() instead of __repr__(). Also slightly changed related-object DoesNotExist exception message to use repr instead of str

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3075 bcc190cf-cafb-0310-a4f2-bffc1f526a37