Send errors to stderr instead of stdio

[barnabasJ]

a general thought I had when going through this was if issues should be printed to stderr instead of studio

@barnabasJ Good thought, but it's unfortunately a little tricky such that I don't want to include that change in this PR. > Maybe worth opening an issue?

The two problems are:

1. We use Mix.shell() to print, but we can't use Mix.shell().error(...) because we do some custom formatting, and it would override everything, making it all red.
2. There isn't a guarantee that Mix.shell() is Mix.Shell.IO, but we could check whether it is, and in that case just do an IO.puts(:stderr, ...) for error cases. If Mix.shell() isn't a Mix.Shell.IO, we could just use the error function of whatever it is.

That's just enough change that I don't want to shoehorn it into this PR, but I think it's a reasonable think to address (or consider addressing) down the line.

Originally posted by @zachallaun in #148 (comment)