def test_update_download_error_message(self):
        """The update_download() method can update a download's error
        message."""
        user = self.gw.create_or_update_user(
            'username', visible_name='Visible Name', max_storage_bytes=1)
        udf = self.factory.make_user_volume(
            owner=user._user, path='~/path/name')
        download = self.gw.make_download(
            user.id, udf.id, 'path', 'http://download/url')
        new_download = self.gw.update_download(
            download.owner_id, download.id, error_message='error')
        self.assertEqual(new_download.error_message, 'error')