htlcswitch: change ForwardPackets to return error

As part of the preparation to the switch interceptor feature, this
function is changed  to return error instead of error channel that
is closed automatically.
Returning an error channel has become complex to maintain and
implement when adding more asynchronous flows to the switch.
The change doesn't affect the current behavior which logs the
errors as before.

htlcswitch/mailbox_test.go

@@ -218,16 +218,13 @@ func newMailboxContext(t *testing.T, startTime time.Time,
 }
 
 func (c *mailboxContext) forward(_ chan struct{},
-	pkts ...*htlcPacket) chan error {
+	pkts ...*htlcPacket) error {
 
 	for _, pkt := range pkts {
 		c.forwards <- pkt
 	}
 
-	errChan := make(chan error)
-	close(errChan)
-
-	return errChan
+	return nil
 }
 
 func (c *mailboxContext) sendAdds(start, num int) []*htlcPacket {
@@ -555,12 +552,8 @@ func TestMailOrchestrator(t *testing.T) {
 			}, nil
 		},
 		forwardPackets: func(_ chan struct{},
-			pkts ...*htlcPacket) chan error {
-			// Close the channel immediately so the goroutine
-			// logging errors can exit.
-			errChan := make(chan error)
-			close(errChan)
-			return errChan
+			pkts ...*htlcPacket) error {
+			return nil
 		},
 		clock:  clock.NewTestClock(time.Now()),
 		expiry: testExpiry,