I created a custom integrator that calls PL/SQL API procedure. It is just updating project API. Strange part is that when I set interface API returns to "Error message" it works fine but if I set it to "FND message code" it starts to fail. I believe those two options shouldn't have any difference when it comes to the actual processing but the end result is different for some reason. Version I am using is 12.1.3
Does anyone know why it's acting like this or any other ways to debug between those two?