Freshly downloaded blobs don't appear until page reload

**What's the problem you want solved?** When we find a blob we don't have, we `want()` it, but then we just return an HTTP 404. 

**Is there a solution you'd like to recommend?** It would be better to `want()` it and then wait for a bit for the blob to get downloaded, and if we receive the blob then we should display it.