Consistent errors / return values for blob getters

[gmaclennan]

Description

blobStore.get() does not consistently handle errors. Specifically, if data is not available, then sometimes it will return null and sometimes it will throw an error.

I think rather than returning null at all, it should always throw an error when data is not available and when it is not found.

It may be difficult / impossible to tell the difference between "not found" and "not downloaded" when the blob db has not replicated.

Tasks

• Consistently throw an error for either not found blobs or not-downloaded blobs

[EvanHahn]

I think this is done because:

1. The code makes it look that way:
   https://github.com/digidem/mapeo-core-next/blob/30e637ba74168d106e17d7b3de349abd97f51c74/src/blob-store/index.js#L99-L101
2. It was checked off in this GitHub comment
3. I see it in 481c991

Closing as completed. Let me know if that's wrong and I'll reopen!