Redirect `atsfmt`'s errors to nowhere.

[kindaro]

We want automatic formatter to be silent. If it has an issue with the code, it can either silently fix it or ignore; it is the job of Syntastic to talk to us.

Writing to stderr seems to break Vim's rendering on some terminals.

[kindaro]

I could not create an issue — it seems you have disabled that feature. I hope this little patch speaks for itself then. Please comment if not.

For motivation, see vmchale/atspkg#13.