lnwire: create distinct type for UpdateFailHTLC failure reason

This commit creates a distint type for the opaque failure reason within
the UpdateFailHTLC message. This new type is needed as this is the only
variable length byte slice within the protocol and therefore requires a
length prefix in order to serialize/deserialize properly.

lnwire/lnwire.go

@@ -72,12 +72,6 @@ func writeElement(w io.Writer, element interface{}) error {
 		if _, err := w.Write(b[:]); err != nil {
 			return err
 		}
-	case FailCode:
-		var b [2]byte
-		binary.BigEndian.PutUint16(b[:], uint16(e))
-		if _, err := w.Write(b[:]); err != nil {
-			return err
-		}
 	case ErrorCode:
 		var b [2]byte
 		binary.BigEndian.PutUint16(b[:], uint16(e))
@@ -170,6 +164,16 @@ func writeElement(w io.Writer, element interface{}) error {
 		if _, err := w.Write(b[:]); err != nil {
 			return err
 		}
+	case OpaqueReason:
+		var l [2]byte
+		binary.BigEndian.PutUint16(l[:], uint16(len(e)))
+		if _, err := w.Write(l[:]); err != nil {
+			return err
+		}
+
+		if _, err := w.Write(e[:]); err != nil {
+			return err
+		}
 	case []byte:
 		if _, err := w.Write(e[:]); err != nil {
 			return err
@@ -339,12 +343,6 @@ func readElement(r io.Reader, element interface{}) error {
 			return err
 		}
 		*e = b[0]
-	case *FailCode:
-		var b [2]byte
-		if _, err := io.ReadFull(r, b[:]); err != nil {
-			return err
-		}
-		*e = FailCode(binary.BigEndian.Uint16(b[:]))
 	case *uint16:
 		var b [2]byte
 		if _, err := io.ReadFull(r, b[:]); err != nil {
@@ -458,6 +456,17 @@ func readElement(r io.Reader, element interface{}) error {
 		}
 		*e = wire.BitcoinNet(binary.BigEndian.Uint32(b[:]))
 		return nil
+	case *OpaqueReason:
+		var l [2]byte
+		if _, err := io.ReadFull(r, l[:]); err != nil {
+			return err
+		}
+		reasonLen := binary.BigEndian.Uint16(l[:])
+
+		*e = OpaqueReason(make([]byte, reasonLen))
+		if _, err := io.ReadFull(r, *e); err != nil {
+			return err
+		}
 	case []byte:
 		if _, err := io.ReadFull(r, e); err != nil {
 			return err