CLI: stderr rather than stdout for error println!s

[marcua]

Consider using the env_logger in server.rs (or removing that package if you don't end up needing it).

[ghost]

If you want to keep usng native rust, I want to remind the existance of eprintln! macro since it is just a printo to stderr

[marcua]

Thank you @AlieNeon! I suspect even eprintln! will be better than what we have now, and will likely start there before taking on more complexity. Welcome to ayb! :)