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

[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!

[mmstick]

Thanks! The reason they were in use was because Ion began development at a time when the eprintln macro did not exist, so there's a mixture of eprintln and writeln.