R1:63acd90cefaf

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…