return err for RemoveHost if hostid does not exist

DEMO:
```
# plu@plu-9: serviced host list
ID		POOL		NAME	ADDR		CORES	MEM		NETWORK
570a276e	default		plu-9	172.17.42.1	12	33659494400	172.17.0.0/255.255.0.0

# plu@plu-9: serviced host remove 570a276e
570a276e

# plu@plu-9: serviced host list
no hosts found

# plu@plu-9: serviced host remove 570a276e
I1112 12:13:13.079290 13533 clients.go:94] rpc error, resetting cached client: HostID 570a276e does not exist
570a276e: HostID 570a276e does not exist
```