Fail explicitly instead of continuing with a warning if "phd.user" is misconfigured

Summary: Fixes T6806. We haven't seen users having issues with phd.user in a very long time.

Test Plan:

• Configured daemons to run as notepriestley, got a well-explained exception.
• Configured daemons to run as epriestley, got a clean start.
• Configured daemons with phd.user=null, got a clean start.

Reviewers: chad, areitz

Reviewed By: areitz

Subscribers: areitz

Maniphest Tasks: T6806

Differential Revision: https://secure.phabricator.com/D15726