At this point, I've gone through all the assets, and the only ones
without manifests are:
1. The ones that truly have no manifest yet (that we know of)
2. The ones where execution happened to time out
I think the 5-second timeout is a very reasonable default for starting
the backfill, in a way that prioritizes moving forward; but now that we
have most things, I'd rather be able to re-run it with a more generous
timeout. So here we are!