R1:b98d46ce7d28

Resurrect setup check for cluster.mailers

Summary:
D19940 removed this file entirely, which has led to at least one user who was unsure how to proceed now that `cluster.mailers` is required for outbound mail: https://discourse.phabricator-community.org/t/invalid-argument-supplied-for-foreach-phabricatormetamtamail-php/2287

This isn't //always// a setup issue for installs that don't care about sending mail, but this at least this gives a sporting chance to users who don't follow the changelogs.

Also, I'm…