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