client: return correct error for 50x response

etcd always returns 500/503 response when it may have no leader.
So we should log the other 50x response in a normal way.

This helps to log correctly when discovery meets 504 error. Before this
PR, it logs like this:

```
18:31:58 etcd2 | 2015/08/4 18:31:58 discovery: error #0: client: etcd
member https://discovery.etcd.io has no leader
18:31:58 etcd2 | 2015/08/4 18:31:58 discovery: waiting for other nodes:
error connecting to https://discovery.etcd.io, retrying in 4s
```

After this PR:

```
22:20:25 etcd2 | 2015/08/4 22:20:25 discovery: error #0: client: etcd
member https://discovery.etcd.io returns server error [Gateway Timeout]
22:20:25 etcd2 | 2015/08/4 22:20:25 discovery: waiting for other nodes:
error connecting to https://discovery.etcd.io, retrying in 4s
```

client/client.go

@@ -245,8 +245,13 @@ func (c *httpClusterClient) Do(ctx context.Context, act httpAction) (*http.Respo
 			continue
 		}
 		if resp.StatusCode/100 == 5 {
-			// TODO: make sure this is a no leader response
-			cerr.Errors = append(cerr.Errors, fmt.Errorf("client: etcd member %s has no leader", eps[k].String()))
+			switch resp.StatusCode {
+			case http.StatusInternalServerError, http.StatusServiceUnavailable:
+				// TODO: make sure this is a no leader response
+				cerr.Errors = append(cerr.Errors, fmt.Errorf("client: etcd member %s has no leader", eps[k].String()))
+			default:
+				cerr.Errors = append(cerr.Errors, fmt.Errorf("client: etcd member %s returns server error [%s]", eps[k].String(), http.StatusText(resp.StatusCode)))
+			}
 			continue
 		}
 		if k != pinned {