htlcswitch: always assume an onion error for malformed htlc failures

Previously a temporary channel failure was returning for unexpected
malformed htlc failures. This is not what we want to communicate to the
sender, because the sender may apply a penalty to us only.

Returning the temporary channel failure is especially problematic if we
ourselves are the sender and the malformed htlc failure comes from our
direct peer. When interpretating the failure, we aren't able to
distinguish anymore between our channel not having enough balance and
our peer sending an unexpected failure back.

lntest/itest/lnd_test.go

@@ -3570,7 +3570,7 @@ func testSphinxReplayPersistence(net *lntest.NetworkHarness, t *harnessTest) {
 
 	// Construct the response we expect after sending a duplicate packet
 	// that fails due to sphinx replay detection.
-	replayErr := "TemporaryChannelFailure"
+	replayErr := "InvalidOnionKey"
 	if !strings.Contains(resp.PaymentError, replayErr) {
 		t.Fatalf("received payment error: %v, expected %v",
 			resp.PaymentError, replayErr)