Debugger: Lua - Fix CTD if callback raises non-string error

[HertzDevil]

Lua's error function is allowed to raise any object as an error, not just strings; when this happens, lua_tostring returns a null pointer in case of incompatible arguments, and Mesen crashes immediately because std::string cannot take a null pointer. Using luaL_tolstring instead will call the Lua tostring, ensuring there is always a valid string message.