R1:f146f4577ea7

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…