Config / setup - check for ONLY_FULL_GROUP_BY and warn the user

Summary: Fixes T6243.

Test Plan: simulated having this on by adding 'ONLY_FULL_GROUP_BY' to the $modes array. verified the error message looked nice {F233954}.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6243

Differential Revision: https://secure.phabricator.com/D10856