Add logs to healthcheck handler

Kubernetes probes don't use nor log the reponse body of health
endpoints. This means that for some reason a nats node running in
Kubernetes becomes on a Not Ready state we won't have a way to know why
other than to manually access the cluster and call the /healthz endpoint
manually and see the error.

This change adds an error log so we can observe what is going wrong with
a nats node that is not ready.

Signed-off-by: Samuel Torres <samuel.torres@form3.tech>

server/monitor.go

@@ -2768,6 +2768,7 @@ func (s *Server) HandleHealthz(w http.ResponseWriter, r *http.Request) {
 
 	hs := s.healthz()
 	if hs.Error != _EMPTY_ {
+		s.Warnf("Healthcheck failed: %q", hs.Error)
 		w.WriteHeader(http.StatusServiceUnavailable)
 	}
 	b, err := json.Marshal(hs)