'Failback host should raise exception when failed.'

@mock.patch.object(replication.Replication, 'start_host_pprc_failback')
def test_failback_host_failed(self, mock_start_host_pprc_failback):

    self.configuration.replication_device = [TEST_REPLICATION_DEVICE]
    self.driver = FakeDS8KProxy(self.storage_info, self.logger, self.exception, self, TEST_TARGET_DS8K_IP)
    self.driver.setup(self.ctxt)
    vol_type = volume_types.create(self.ctxt, 'VOL_TYPE', {'replication_enabled': '<is>    True'})
    location = six.text_type({'vol_hex_id': TEST_VOLUME_ID})
    data = json.dumps({'default': {'vol_hex_id': TEST_VOLUME_ID}})
    volume = self._create_volume(volume_type_id=vol_type.id, provider_location=location, replication_driver_data=data)
    mock_start_host_pprc_failback.side_effect = restclient.APIException('failed    to    do    failback.')
    self.assertRaises(exception.UnableToFailOver, self.driver.failover_host, self.ctxt, [volume], 'default', [])
