Integration test: 404 for not existing Phabricator door

Summary:
Phabricator doors must be declared in credentials.json. We ensure
if a non existing door is used, a 404 error is returned.

This will achieve full coverage of the PhabricatorGateController.

Test Plan: Run tests

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D249