htlcswitch: add new default case when handling UpdateFailMalformedHLTC

In this commit, we modify the existing logic to handle
UpdateFailMalformedHLTC message from an incoming peer. Rather than fail
the Chanel if they give us an invalid failure code, we’ll instead treat
it as a temporary channel failure so we can continue to forward the
error.

htlcswitch/link.go

@@ -859,10 +859,8 @@ func (l *channelLink) handleUpstreamMsg(msg lnwire.Message) {
 				OnionSHA256: msg.ShaOnionBlob,
 			}
 		default:
-			// TODO(roasbeef): fail channel here?
-			log.Errorf("unable to understand code of received " +
-				"malformed error")
-			return
+			log.Errorf("Unknown failure code: %v", msg.FailureCode)
+			failure = &lnwire.FailTemporaryChannelFailure{}
 		}
 
 		// With the error parsed, we'll convert the into it's opaque