Guard against fatal errors re. eternal recursion

Problem/Motivation

Steps to reproduce

Check what happens when rendering entity A that references entity A.

or: entity A that references entity B, and entity B that references entity A.

It's "the user's fault" if they configure the CE display this way, but I think we should at least prevent ugly fatal errors. Maybe log.

Proposed resolution

Some anti-recursion killswitch? In the specific formatter?

Remaining tasks

First: check what happens. (I haven't yet.)