htlcswitch: implement stfu response

htlcswitch: use quiescer SendOwedStfu method in link stfu implementation

htlcswitch/linkfailure.go

@@ -51,6 +51,12 @@ const (
 	// circuit map. This is non-fatal and will resolve itself (usually
 	// within several minutes).
 	ErrCircuitError
+
+	// ErrStfuViolation indicates that the quiescence protocol has been
+	// violated, either because Stfu has been sent/received at an invalid
+	// time, or that an update has been sent/received while the channel is
+	// quiesced.
+	ErrStfuViolation
 )
 
 // LinkFailureAction is an enum-like type that describes the action that should
@@ -122,6 +128,8 @@ func (e LinkFailureError) Error() string {
 		return "unable to resume channel, recovery required"
 	case ErrCircuitError:
 		return "non-fatal circuit map error"
+	case ErrStfuViolation:
+		return "quiescence protocol executed improperly"
 	default:
 		return "unknown error"
 	}