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 
We now want to allow registration of unhealthy node to apiserver; if error is proprigate to apiserver as a raw error, then apiserver will panic and return the raw error to user (the error is re-interprete by apiserver).  User may want to retry, but the node is alway successfully registered (really, it is how it works now).

Yeah, the solution is not ideal.  That's why I'm going to remove this and have a node controller handles health check.