multi: add ability to read warning messages and log on link level

Warning messages are intended to add "softer" failure modes for peers,
so to start with we simply log the warnings sent to us. While we "may"
disconnect from the peer according to the spec, we start with the least
extreme option (which is also not a change in behavior because
previously we'd just log that we received an unknown odd message).

peer/brontide.go

@@ -1455,6 +1455,10 @@ out:
 				break out
 			}
 
+		case *lnwire.Warning:
+			targetChan = msg.ChanID
+			isLinkUpdate = p.handleError(&msg.Error)
+
 		case *lnwire.Error:
 			targetChan = msg.ChanID
 			isLinkUpdate = p.handleError(msg)
@@ -1688,6 +1692,9 @@ func messageSummary(msg lnwire.Message) string {
 		return fmt.Sprintf("chan_id=%v, id=%v, fail_code=%v",
 			msg.ChanID, msg.ID, msg.FailureCode)
 
+	case *lnwire.Warning:
+		return fmt.Sprintf("%v", msg.Error.Error())
+
 	case *lnwire.Error:
 		return fmt.Sprintf("%v", msg.Error())