Conceptualize how to deal with incompletely propagated bill data

[zupzup]

Currently, it's possible that one gets one of the two files (bill / bill keys) and then fails, because it expects the other to be there. The same goes for attachments on bills - the bill might be there, but not the attachments yet

In that case, we need to attempt to fetch (in the background - and retry) the missing file and somehow return to frontend, that we don't have all the data (yet).