Improve error message for "phd stop" with bad PID
Summary: "phd launch ircbot" works but "phd stop ircbot" gives you a potentially
confusing message. Improve messaging.
Test Plan: Ran "phd stop ircbot", "phd stop 87888" (actual PID)
Reviewers: btrahan, jungejason
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T791
Differential Revision: https://secure.phabricator.com/D1457