Configuration - re-jigger how we handle bad configuration files

Summary: just explicitly check if the file doesn't exist *first*, and then do the standard include thing with the more generic error if that doesn't work. Fixes T6255.

Test Plan: re-started apache and phabricator still worked; will ask csilvers to give it a whirl too

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6255

Differential Revision: https://secure.phabricator.com/D10643