Makes franka::Error trivially copyable

[samzapo]

Using a std::atomic<franka::RobotState> can permit users users to communicate a robot's state efficiently between threads. Trivially copyable is a special property required by values stored in std::atomic, but it requires that all copy assignment and constructors for an object and all of its members be defined as default. The franka::RobotState struct is very close to meeting this requirement, all of its members are trivially copyable except for Error- which can readily meet the requirement.

This PR makes the necessary changes for std::atomic to hold a franka::RobotState.