Allow dashboard panels to detect rendering cycles and arrest stack overflows

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