Differentiate between "no pygmetnize" and "nonworking pygmentize" during setup

Summary: Fixes T6210. The current messaging may be confusing if `pygmentize` is available but broken.

Test Plan: Faked the binary names and hit the errors, which seemed helpful.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6210

Differential Revision: https://secure.phabricator.com/D10626