Still need to do sleep/backoff, etc. Hoping to remove NodataReader too.

When an `open` fails and we'd return nodata instead, we don't cache that anymore. So if the asset doesn't exist, we'll try to open it for every chunk. Unclear how much of a performance impact this will have. It's probably not ideal, because combining it with retries could be nice: retry errors first, then give up and use nodata if they persist?

got lost in #221, which was a reasonable change, but turns out there was still a test importing it

TODO:
- sleep & backoff
- figure out how to handle sleep in tests