etcdctl/cluster_health: improve output if failed to get leader stats

When failing to get leader stats, it said 'cluster is unhealthy' before.
This is confusing when it cannot get stats because advertised client urls
are set wrong and the cluster is healthy.

etcdctl/command/cluster_health.go

@@ -46,9 +46,10 @@ func handleClusterHealth(c *cli.Context) {
 	}
 
 	// do we have a leader?
-	ep, ls0, err := getLeaderStats(tr, client.GetCluster())
+	cl := client.GetCluster()
+	ep, ls0, err := getLeaderStats(tr, cl)
 	if err != nil {
-		fmt.Println("cluster is unhealthy")
+		fmt.Println("cluster may be unhealthy: failed to connect", cl)
 		os.Exit(1)
 	}