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/link.go

@@ -135,10 +135,10 @@ type ChannelLinkConfig struct {
 	Switch *Switch
 
 	// ForwardPackets attempts to forward the batch of htlcs through the
-	// switch, any failed packets will be returned to the provided
-	// ChannelLink. The link's quit signal should be provided to allow
+	// switch. The function returns and error in case it fails to send one or
+	// more packets. The link's quit signal should be provided to allow
 	// cancellation of forwarding during link shutdown.
-	ForwardPackets func(chan struct{}, ...*htlcPacket) chan error
+	ForwardPackets func(chan struct{}, ...*htlcPacket) error
 
 	// DecodeHopIterators facilitates batched decoding of HTLC Sphinx onion
 	// blobs, which are then used to inform how to forward an HTLC.
@@ -2971,8 +2971,10 @@ func (l *channelLink) forwardBatch(packets ...*htlcPacket) {
 		filteredPkts = append(filteredPkts, pkt)
 	}
 
-	errChan := l.cfg.ForwardPackets(l.quit, filteredPkts...)
-	go handleBatchFwdErrs(errChan, l.log)
+	if err := l.cfg.ForwardPackets(l.quit, filteredPkts...); err != nil {
+		log.Errorf("Unhandled error while reforwarding htlc "+
+			"settle/fail over htlcswitch: %v", err)
+	}
 }
 
 // sendHTLCError functions cancels HTLC and send cancel message back to the