R1:223d7b84dd18

Recover more gracefully when favicon configuration points at a corrupt/damaged file

Summary:
Ref T13103. Locally, I managed to break the data for a bunch of files by doing `git clean -df` in a working copy that I'd updated to a commit from many many years ago. Since `conf/local.json` wasn't on the gitignore list many years ago, this removed it, and I lost my encryption keyring.

I've symlinked my local config to a version-controlled file now to avoid this specific type of creative self-sabotage in the…