Say nothing serves a call only for the bus's own no-responders, never for a tool's error that mentions 503 (issue 1529)

node-tools/internal/bus/bus.go

@@ -708,12 +708,16 @@ var HandoverPause = 3 * time.Second
 // (mesh-controller internal/link, `"retry": "handing-over"`).
 const RetryHandingOver = "handing-over"
 
+// ErrNoResponders is the bus's own refusal: nothing subscribes to the subject asked. It is told apart
+// from a tool's own error by errors.Is, never by its text (a tool may say "503" itself, issue 1529).
+var ErrNoResponders = errors.New("503 no responders")
+
 // askOnce asks once, and says whether the answer was a refusal marked to be asked again.
 func (c *Conn) askOnce(subject string, data []byte) (Answered, bool, error) {
 	msg, err := c.nc.Request(subject, data, RequestTimeout)
 	if err != nil {
 		if errors.Is(err, nats.ErrNoResponders) {
-			return Answered{}, false, errors.New("503 no responders")
+			return Answered{}, false, ErrNoResponders
 		}
 		if errors.Is(err, nats.ErrTimeout) {
 			return Answered{}, false, errors.New("timeout")