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