Modify Ok and Error

[threeal]

modify how ok and error works by:

• moving ok and error from res::internal to res namespace.
• force to create a new res::Ok instance instead of using constant (for simplicity).
• replace res::ErrMsgPtr that was using std::shared_ptr<std::string> to just using std::string (for simplicity).
• create res::Err struct to wrap error data and force to create a new res::Err instance instead of just passing the error message.