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/bus.go

@@ -206,8 +206,9 @@ func (b OverNATS) AskTool(ctx context.Context, module, tool string, args []byte,
 	if err != nil {
 		if errors.Is(err, nats.ErrNoResponders) {
 			// Said at once rather than after the whole wait: nothing is subscribed to that
-			// subject, which is a different fact from a tool being slow.
-			return nil, fmt.Errorf("nothing serves %s.%s", module, tool)
+			// subject, which is a different fact from a tool being slow. Said as ErrNothingServes, so an
+			// asker can tell a tool not served now from one that refused (novox/hq issue 486).
+			return nil, saidAs{fmt.Sprintf("nothing serves %s.%s", module, tool), ErrNothingServes}
 		}
 		return nil, fmt.Errorf("asking %s.%s: %w", module, tool, err)
 	}