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 
I actually think it's vastly simpler to just count any failure as an error, and have a rule about N errors in a row (or out of last M checks) means you're unhealthy.

The other alternative means we have to make an ErrUnknown and deal with uncertainty through the rest of the stack-- certainly a better model of reality, but also much more complicated and with questionable additional benefit.