routing: modify TestSendPaymentRouteFailureFallback to use non-critical error

In this commit we modify the existing
TestSendPaymentRouteFailureFallback to use a non-critical error aside
from FailChannelDisabled. This is necessary as the behavior of the
current error handling can fail due to us sending in a nil error.

routing/router_test.go

@@ -179,7 +179,7 @@ func TestSendPaymentRouteFailureFallback(t *testing.T) {
 		_ *lnwire.UpdateAddHTLC, _ *sphinx.Circuit) ([32]byte, error) {
 
 		if ctx.aliases["luoji"].IsEqual(n) {
-			return [32]byte{}, &lnwire.FailChannelDisabled{}
+			return [32]byte{}, &lnwire.FailUnknownNextPeer{}
 		}
 
 		return preImage, nil