[Format] inserts output into editor (in some cases)

[respeccing]

UPDATE: This happens because format depends on rustfmt's returning non-zero exit code when errors happened.

I've tried reverting PR #191 (just to make sure) but the issue remains. So this must've been there from before. I'll see if I can figure out a fix...this is irrelevant

To test, press [Format] on this code:
https://play.rust-lang.org/?gist=31a52c6d6288b16eda5316f014c94a09&version=stable&backtrace=0

before:

after:

EDIT: To test when it works:
put only this in rust-playpen then press Format:
a backtick character
or
d
(yep, 1 char things)
Now output isn't inserted into editor, because exit code is non-zero in both above cases.