[1.9.x] Fixed typo in GenericRelatedObjectManager.add() error message.

                    if obj._state.adding or obj._state.db != db:

                        raise ValueError(

                            "%r instance isn't saved. Use bulk=False or save "

                            "the object first. but must be." % obj

                            "the object first." % obj

                        )

                    check_and_update_obj(obj)

                    pks.append(obj.pk)