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 
No, I don't think ErrNotHealthy is appropriate in all cases.  What if the tcp stream is broken.  I think there is a class of errors where you actually don't know, and you shouldn't return something definitive.