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/mock_test.go

@@ -35,12 +35,12 @@ func (m *mockPaymentAttemptDispatcher) SendHTLC(firstHop lnwire.ShortChannelID,
 	var result *htlcswitch.PaymentResult
 	preimage, err := m.onPayment(firstHop)
 	if err != nil {
-		fwdErr, ok := err.(*htlcswitch.ForwardingError)
+		rtErr, ok := err.(htlcswitch.ClearTextError)
 		if !ok {
 			return err
 		}
 		result = &htlcswitch.PaymentResult{
-			Error: fwdErr,
+			Error: rtErr,
 		}
 	} else {
 		result = &htlcswitch.PaymentResult{Preimage: preimage}