multi: stop casting peer warning messages as errors

Split the logic for processing `error` and `warning` messages from our
peers.

lnwire/warning.go

@@ -1,16 +1,70 @@
 package lnwire
 
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+// WarningData is a set of bytes associated with a particular sent warning. A
+// receiving node SHOULD only print out data verbatim if the string is composed
+// solely of printable ASCII characters. For reference, the printable character
+// set includes byte values 32 through 127 inclusive.
+type WarningData []byte
+
+// Warning is used to express non-critical errors in the protocol, providing
+// a "soft" way for nodes to communicate failures.
+type Warning struct {
+	// ChanID references the active channel in which the warning occurred
+	// within. If the ChanID is all zeros, then this warning applies to the
+	// entire established connection.
+	ChanID ChannelID
+
+	// Data is the attached warning data that describes the exact failure
+	// which caused the warning message to be sent.
+	Data WarningData
+}
+
 // 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
+// NewWarning creates a new Warning message.
+func NewWarning() *Warning {
+	return &Warning{}
+}
+
+// Warning returns the string representation to Warning.
+func (c *Warning) Warning() string {
+	errMsg := "non-ascii data"
+	if isASCII(c.Data) {
+		errMsg = string(c.Data)
+	}
+
+	return fmt.Sprintf("chan_id=%v, err=%v", c.ChanID, errMsg)
+}
+
+// Decode deserializes a serialized Warning message stored in the passed
+// io.Reader observing the specified protocol version.
+//
+// This is part of the lnwire.Message interface.
+func (c *Warning) Decode(r io.Reader, pver uint32) error {
+	return ReadElements(r,
+		&c.ChanID,
+		&c.Data,
+	)
+}
+
+// Encode serializes the target Warning into the passed io.Writer observing the
+// protocol version specified.
+//
+// This is part of the lnwire.Message interface.
+func (c *Warning) Encode(w *bytes.Buffer, pver uint32) error {
+	if err := WriteBytes(w, c.ChanID[:]); err != nil {
+		return err
+	}
+
+	return WriteWarningData(w, c.Data)
 }
 
 // MsgType returns the integer uniquely identifying an Warning message on the