server: return error from sendPeerMessages

This commit adds a return error to sendPeerMessages,
making it possible to observe if a message sent to
a peer fails or succeeds.

server.go

@@ -950,15 +950,27 @@ func (s *server) sendToPeer(target *btcec.PublicKey,
 		return err
 	}
 
-	s.sendPeerMessages(targetPeer, msgs, nil)
-
+	// Send messages to the peer and return any error from
+	// sending a message.
+	errChans := s.sendPeerMessages(targetPeer, msgs, nil)
+	for _, errChan := range errChans {
+		select {
+		case err := <-errChan:
+			return err
+		case <-s.quit:
+			return ErrServerShuttingDown
+		}
+	}
 	return nil
 }
 
 // sendPeerMessages enqueues a list of messages into the outgoingQueue of the
 // `targetPeer`.  This method supports additional broadcast-level
 // synchronization by using the additional `wg` to coordinate a particular
-// broadcast.
+// broadcast. Since this method will wait for the return error from sending
+// each message, it should be run as a goroutine (see comment below) and
+// the error ignored if used for broadcasting messages, where the result
+// from sending the messages is not of importance.
 //
 // NOTE: This method must be invoked with a non-nil `wg` if it is spawned as a
 // go routine--both `wg` and the server's WaitGroup should be incremented
@@ -968,7 +980,7 @@ func (s *server) sendToPeer(target *btcec.PublicKey,
 func (s *server) sendPeerMessages(
 	targetPeer *peer,
 	msgs []lnwire.Message,
-	wg *sync.WaitGroup) {
+	wg *sync.WaitGroup) []chan error {
 
 	// If a WaitGroup is provided, we assume that this method was spawned
 	// as a go routine, and that it is being tracked by both the server's
@@ -981,9 +993,17 @@ func (s *server) sendPeerMessages(
 		defer wg.Done()
 	}
 
+	// We queue each message, creating a slice of error channels that
+	// can be inspected after every message is successfully added to
+	// the queue.
+	var errChans []chan error
 	for _, msg := range msgs {
-		targetPeer.queueMsg(msg, nil)
+		errChan := make(chan error, 1)
+		targetPeer.queueMsg(msg, errChan)
+		errChans = append(errChans, errChan)
 	}
+
+	return errChans
 }
 
 // FindPeer will return the peer that corresponds to the passed in public key.