htlcswitch: use LinkError for internal errors

Update the ChannelLink interface to specifically
return the LinkError struct. This error implements
the ClearTextError interface, so will be picked
up as a routing realted error by the router.

With LinkErrors implemented, the switch now
returns a LinkError for all failures on our
incoming/outgoing link and ForwardingError when
the failure occurs down the line.

htlcswitch/failure.go

@@ -33,6 +33,9 @@ type LinkError struct {
 	// know the failure type for failures which occur at our own
 	// node.
 	msg lnwire.FailureMessage
+
+	// FailureDetail enriches the wire error with additional information.
+	FailureDetail
 }
 
 // NewLinkError returns a LinkError with the failure message provided.
@@ -42,6 +45,17 @@ func NewLinkError(msg lnwire.FailureMessage) *LinkError {
 	return &LinkError{msg: msg}
 }
 
+// NewDetailedLinkError returns a link error that enriches a wire message with
+// a failure detail.
+func NewDetailedLinkError(msg lnwire.FailureMessage,
+	detail FailureDetail) *LinkError {
+
+	return &LinkError{
+		msg:           msg,
+		FailureDetail: detail,
+	}
+}
+
 // WireMessage extracts a valid wire failure message from an internal
 // error which may contain additional metadata (which should not be
 // exposed to the network). This value should never be nil for LinkErrors,
@@ -56,7 +70,13 @@ func (l *LinkError) WireMessage() lnwire.FailureMessage {
 //
 // Note this is part of the ClearTextError interface.
 func (l *LinkError) Error() string {
-	return l.msg.Error()
+	// If the link error has no failure detail, return the wire message's
+	// error.
+	if l.FailureDetail == FailureDetailNone {
+		return l.msg.Error()
+	}
+
+	return fmt.Sprintf("%v: %v", l.msg.Error(), l.FailureDetail)
 }
 
 // ForwardingError wraps an lnwire.FailureMessage in a struct that also