htlcswitch: add LinkError implementation of ClearTextError

This change introduces a LinkError implementation
of the ClearTextError interface. This error is intended
to represent failures which occur on our incoming and
outgoing link when sending, receiving and forwarding
htlcs. Paired with ForwardingError, which is represents
failures that did not occur at our node, this error
covers all non-opaque errors that the switch experiences.

htlcswitch/failure.go

@@ -25,6 +25,40 @@ type ClearTextError interface {
 	WireMessage() lnwire.FailureMessage
 }
 
+// LinkError is an implementation of the ClearTextError interface which
+// represents failures that occur on our incoming or outgoing link.
+type LinkError struct {
+	// msg returns the wire failure associated with the error.
+	// This value should *not* be nil, because we should always
+	// know the failure type for failures which occur at our own
+	// node.
+	msg lnwire.FailureMessage
+}
+
+// NewLinkError returns a LinkError with the failure message provided.
+// The failure message provided should *not* be nil, because we should
+// always know the failure type for failures which occur at our own node.
+func NewLinkError(msg lnwire.FailureMessage) *LinkError {
+	return &LinkError{msg: msg}
+}
+
+// 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,
+// because we are the ones failing the htlc.
+//
+// Note this is part of the ClearTextError interface.
+func (l *LinkError) WireMessage() lnwire.FailureMessage {
+	return l.msg
+}
+
+// Error returns the string representation of a link error.
+//
+// Note this is part of the ClearTextError interface.
+func (l *LinkError) Error() string {
+	return l.msg.Error()
+}
+
 // ForwardingError wraps an lnwire.FailureMessage in a struct that also
 // includes the source of the error.
 type ForwardingError struct {