include the likely cause of the error in the error message

client.go

@@ -188,7 +188,11 @@ func (d *Dialer) DialContext(ctx context.Context, urlStr string, requestHeader h
 
 	for _, proto := range d.TLSClientConfig.NextProtos {
 		if proto != "http/1.1" {
-			return nil, nil, fmt.Errorf("protocol %q is currently not supported", proto)
+			return nil, nil, fmt.Errorf(
+				`protocol %q was given but is not supported;
+				sharing tls.Config with net/http Transport can cause this error`,
+				proto,
+			)
 		}
 	}