Exercises

1. Implement another function in LuaExecutor to call a Lua function with two parameters and two return values. Try to use different Lua data types.
2. In LuaExecutor, we are using std::cerr to print error messages to the console. So far, the callers cannot get the error state. Design an interface to notify failures. You can pass an implementation of this interface in the LuaExecutor constructor.