[1.6.x] Fixed #20791 -- Reworded ForeignKey default error message

Backport of 311c1d2848 from master.

django/db/models/fields/related.py

@@ -1125,7 +1125,7 @@ class ForeignObject(RelatedField):
 class ForeignKey(ForeignObject):
     empty_strings_allowed = False
     default_error_messages = {
-        'invalid': _('Model %(model)s with pk %(pk)r does not exist.')
+        'invalid': _('%(model)s instance with pk %(pk)r does not exist.')
     }
     description = _("Foreign Key (type determined by related field)")