Summary:
Ref T4986. Ref T4983. Panels will soon be able to contain other panels, either via Remarkup (`{W1}`) or maybe through new types of meta-panels.
Allow panels to detect that they are being rendered very deeply and/or within themselves.
Test Plan: Faked some errors, got failed panel renders. Since panels can't //really// contain other panels yet, this doesn't really have an impact.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: chad, epriestley
Maniphest Tasks: T4983, T4986
Differential Revision: https://secure.phabricator.com/D9140
63acd90cef