Return value for LuaEntity.set_driver() and LuaEntity.set_passenger()

[Pi-C]

LuaEntity.set_driver() and LuaEntity.set_passenger() will silently fail if allow_passengers has been set to false in the vehicle prototype. I think it might be useful to get some feedback when running these commands. Returning true if driver or passenger have been successfully set or removed, and false if the prototype doesn't allow for driver/passenger, would seem like a good idea.