channelnotifier+discover+invoices: return error in Stop functions

In order to be consistent with other sub systems an error is now
returned from the Stop functions.
This also allows writing a generic cleanup mechanism to stop all
sub systems in case of a failure.

discovery/gossiper.go

@@ -461,8 +461,9 @@ func (d *AuthenticatedGossiper) start() error {
 }
 
 // Stop signals any active goroutines for a graceful closure.
-func (d *AuthenticatedGossiper) Stop() {
+func (d *AuthenticatedGossiper) Stop() error {
 	d.stopped.Do(d.stop)
+	return nil
 }
 
 func (d *AuthenticatedGossiper) stop() {