writeln!(stderr, ...) -> eprintln!

Created by: AgustinCB

While working in other tasks, I noticed that there's a rather inconsistent way to handle errors. In some places we use eprintln! and in others writeln! to standard error.

It seems like the use of writeln! don't have an strong motive, as the output is always to standard error, the lock is always handled the same than eprintln and there's no obvious (to me) optimization, so I decided to use the more idiomatic eprintln! everywhere.

Let me know if there's seomthing I'm missing!