Wait out a forge not served for a moment when registering a build (novox/hq issue 486)

A registration asked the forge while node-tools re-served after a tool-runner
rollout, heard "nothing serves gitea.gitea_api", and failed a good build's
plan. The bus's not-served answer is now link.ErrNothingServes (and a timeout
link.ErrNoAnswer), and the registration asks again for up to 90 s on those
alone, never on a refusal; past the bound it fails saying how long it waited.

internal/link/ask.go

@@ -16,6 +16,19 @@ type Answer struct {
 	Error  string          `json:"error,omitempty"`
 }
 
+// ErrNoAnswer is a tool asked that did not answer within the asker's wait: nothing refused, and asking again
+// may be answered. With ErrNothingServes, what an asker may wait out (novox/hq issue 486).
+var ErrNoAnswer = errors.New("no answer within the wait")
+
+// saidAs is an error said in its own words that is still the error it is a case of, for errors.Is.
+type saidAs struct {
+	text string
+	is   error
+}
+
+func (e saidAs) Error() string { return e.text }
+func (e saidAs) Unwrap() error { return e.is }
+
 // Ask calls one of a module's tools over the broker and waits for its answer.
 //
 // **The control plane is the way in** (novox/hq 04-ISSUES/049, ADR 0095). A module's broker
@@ -37,16 +50,16 @@ func Ask(ctx context.Context, bus Bus, module, tool string, args json.RawMessage
 	reply, err := bus.AskTool(ctx, module, tool, args, timeout)
 	if err != nil {
 		if errors.Is(err, nats.ErrNoResponders) {
-			return Answer{}, fmt.Errorf(
+			return Answer{}, saidAs{fmt.Sprintf(
 				"nothing serves %s: no runtime has bound %q on the bus. The module is not "+
 					"assigned, its runtime is not up, or it serves no such tool — `status` says "+
-					"whether the machine carrying it has applied", module, key)
+					"whether the machine carrying it has applied", module, key), ErrNothingServes}
 		}
 		if errors.Is(err, context.DeadlineExceeded) || errors.Is(err, nats.ErrTimeout) {
-			return Answer{}, fmt.Errorf(
+			return Answer{}, saidAs{fmt.Sprintf(
 				"%s did not answer within %s. Its runtime serves %q when it is up and has bound "+
 					"the bus — `status` says whether the machine carrying it has applied",
-				module, timeout, key)
+				module, timeout, key), ErrNoAnswer}
 		}
 		return Answer{}, fmt.Errorf("cannot ask %s: %w", key, err)
 	}