htlcswitch+routing: type check on ClearTextError

Update the type check used for checking local payment
failures to check on the ClearTextError interface rather
than on the ForwardingError type. This change prepares
for splitting payment errors up into Link and Forwarding
errors.

routing/router.go

@@ -1910,18 +1910,33 @@ func (r *ChannelRouter) processSendError(paymentID uint64, rt *route.Route,
 
 		return reportFail(nil, nil)
 	}
-	// If an internal, non-forwarding error occurred, we can stop
-	// trying.
-	fErr, ok := sendErr.(*htlcswitch.ForwardingError)
+
+	// If the error is a ClearTextError, we have received a valid wire
+	// failure message, either from our own outgoing link or from a node
+	// down the route. If the error is not related to the propagation of
+	// our payment, we can stop trying because an internal error has
+	// occurred.
+	rtErr, ok := sendErr.(htlcswitch.ClearTextError)
 	if !ok {
 		return &internalErrorReason
 	}
 
-	failureMessage := fErr.WireMessage()
-	failureSourceIdx := fErr.FailureSourceIdx
+	// failureSourceIdx is the index of the node that the failure occurred
+	// at. If the ClearTextError received is not a ForwardingError the
+	// payment error occurred at our node, so we leave this value as 0
+	// to indicate that the failure occurred locally. If the error is a
+	// ForwardingError, it did not originate at our node, so we set
+	// failureSourceIdx to the index of the node where the failure occurred.
+	failureSourceIdx := 0
+	source, ok := rtErr.(*htlcswitch.ForwardingError)
+	if ok {
+		failureSourceIdx = source.FailureSourceIdx
+	}
 
-	// Apply channel update if the error contains one. For unknown
-	// failures, failureMessage is nil.
+	// Extract the wire failure and apply channel update if it contains one.
+	// If we received an unknown failure message from a node along the
+	// route, the failure message will be nil.
+	failureMessage := rtErr.WireMessage()
 	if failureMessage != nil {
 		err := r.tryApplyChannelUpdate(
 			rt, failureSourceIdx, failureMessage,