Return node unhealthy if http request fails

If health check cannot talk to kubelet (http connection fail, instead of 500 code), it will return a connection fail instead of Unhealth status. Part of #2192

@lavalamp 
Yeah, I have the same question as lavalamp.  An error always come with an Unknown status.  

We can define another status if using Unknown is not desirable, but I think health check should return a predefined stutus, instead of a raw error messge from underneath.