Crates that are fetched but not added to the index appear to be lost

This is a slightly different issue than #628 - we _record_ the failure internally, it won't be lost, we just don't display it anywhere but in the logs. So it seems from the outside as if the build was never there, but we can still rerun it with `UPDATE queue SET attempts = 0;`

_Originally posted by @jyn514 in https://github.com/rust-lang/docs.rs/issues/628#issuecomment-660687177_