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/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
+}