display network name when disconnecting network error

Signed-off-by: allencloud <allen.sun@daocloud.io>

daemon/container_operations.go

@@ -808,7 +808,7 @@ func (daemon *Daemon) disconnectFromNetwork(container *container.Container, n li
 	}
 
 	if ep == nil {
-		return fmt.Errorf("container %s is not connected to the network", container.ID)
+		return fmt.Errorf("container %s is not connected to network %s", container.ID, n.Name())
 	}
 
 	if err := ep.Leave(sbox); err != nil {