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 {