htlcswitch: add detailed "insufficient bandwidth" log message

fixes #5250

htlcswitch/link.go

@@ -2344,6 +2344,8 @@ func (l *channelLink) canSendHtlc(policy ForwardingPolicy,
 
 	// Check to see if there is enough balance in this channel.
 	if amt > l.Bandwidth() {
+		l.log.Errorf("insufficient bandwidth to route htlc: %v is "+
+			"larger than %v", amt, l.Bandwidth())
 		failure := l.createFailureWithUpdate(
 			func(upd *lnwire.ChannelUpdate) lnwire.FailureMessage {
 				return lnwire.NewTemporaryChannelFailure(upd)