Validate all components of $PATH configuration

Summary: Fixes T3400. Users are crafty. Attempt to outwit them.

Test Plan: Added all kinds of nonsense to my PATH to hit all the errors. Verified sensible-looking error messages which I couldn't figure out any way to misread or outwit.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3400

Differential Revision: https://secure.phabricator.com/D6318