Serve generic 502 pages from Phabricator Zed instance

Summary:
Phabricator nginx configuration seeks the 502 page
in a directory with the canonical server name.

This is a follow-up for 726fa910fe44.

Test Plan: Browse https://code.zed.dereckson.be/502.htmll

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1971