Create phd PID directory if it doesn't exist, and give the user a meaningful error message if that doesn't work

Summary:
This workflow is needlessly bad right now, make it not terrible.

Also removed some related, unused code.

Test Plan:
Ran 'phd launch nice' with no directory and with a failing mkdir command.

Reviewed By: toulouse
Reviewers: hsb, toulouse, codeblock
CC: aran, toulouse
Differential Revision: 440