Send arcanist error output to STDERR
Send arcanist error output to STDERR

Summary: Currently, arcanist error output is sent to `STDOUT` instead of `STDOUT`. This is annoying because I am running `arc lint --everything --never-apply-patches --output=xml > checkstyle.xml` and the `checkstyle.xml` file is not valid XML.

Test Plan: Forced a linter to throw an exception and ran `arc lint >/dev/null`... saw error output.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13043

julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist