lnwire: add warning message wrapping existing Error message structure

This commit adds Warning messages to lnwire, as introduced in bolts/950.
It does not include reading/writing of warning messages, which will be
covered in followup commits.

lnwire/lnwire_test.go

@@ -964,6 +964,12 @@ func TestLightningWireProtocol(t *testing.T) {
 				return mainScenario(&m)
 			},
 		},
+		{
+			msgType: MsgWarning,
+			scenario: func(m Warning) bool {
+				return mainScenario(&m)
+			},
+		},
 		{
 			msgType: MsgError,
 			scenario: func(m Error) bool {

lnwire/message.go

@@ -22,7 +22,8 @@ type MessageType uint16
 // The currently defined message types within this current version of the
 // Lightning protocol.
 const (
-	MsgInit                    MessageType = 16
+	MsgWarning                 MessageType = 1
+	MsgInit                                = 16
 	MsgError                               = 17
 	MsgPing                                = 18
 	MsgPong                                = 19
@@ -75,6 +76,8 @@ func ErrorPayloadTooLarge(size int) error {
 // String return the string representation of message type.
 func (t MessageType) String() string {
 	switch t {
+	case MsgWarning:
+		return "Warning"
 	case MsgInit:
 		return "Init"
 	case MsgOpenChannel:
@@ -175,6 +178,8 @@ func makeEmptyMessage(msgType MessageType) (Message, error) {
 	var msg Message
 
 	switch msgType {
+	case MsgWarning:
+		msg = &Warning{}
 	case MsgInit:
 		msg = &Init{}
 	case MsgOpenChannel:

lnwire/warning.go

@@ -0,0 +1,22 @@
+package lnwire
+
+// A compile time check to ensure Warning implements the lnwire.Message
+// interface.
+var _ Message = (*Warning)(nil)
+
+// Warning is used to express non-critical errors in the protocol, providing
+// a "soft" way for nodes to communicate failures. Since it has the same
+// structure as errors, warnings simply include an Error so that we can leverage
+// their encode/decode functionality, and over write the MsgType function to
+// differentiate them.
+type Warning struct {
+	Error
+}
+
+// MsgType returns the integer uniquely identifying an Warning message on the
+// wire.
+//
+// This is part of the lnwire.Message interface.
+func (c *Warning) MsgType() MessageType {
+	return MsgWarning
+}