R1:5001aadf4660

Throw a more helpful error for bad Differential actions

Summary: Ref T7611. This should let us figure out the root cause, hopefully.

Test Plan: iiam

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7611

Differential Revision: https://secure.phabricator.com/D12124