Improve Error Handling

[davidsiaw]

Very little error handling currently. When you get nil or nothing it simply segfaults due to NULL referencing.

Should throw exceptions when argument types and stuff don't match back into Lua code

Or throw exceptions in C++ when an error is thrown out of Lua code

[davidsiaw]

This is done now that an error cleanup has happened. All errors are now Lua errors. Failing that, lua_atpanic will be called. This raises an exception in LuaCppInterface