Cache outbound fetch errors

[snarfed]

If we try to fetch an external record and the fetch fails, we should cache that error with a short-lived TTL. Related: #1097

Background: https://fed.brid.gy/bsky/did:plc:tgguyu3xtyrmkfj4xljevyqo followed a ton of people right now (natively in Bluesky), which triggered a bunch of receive tasks. Somehow that user doesn't have an app.bsky.actor.profile/self record, so when we try to fetch their profile, it fails - specifically, getRecord calls to api.bsky.app return 400 - and the receive tasks fail, but we keep trying.