[BUG] Error handling in uploads

[jesmrec]

Comes from #3913 (comment)

When a upload fails for a known issue (check list in link above), the feedback to the user is not descriptive. Almost all errors are labelled with Unknown error instead a friendly or helpful message.

Upload errors are set in two different places: uploads view (persisted) and notification (not persisted). Both should match somehow.

[abelgardep]

It's a little bit strange. I have tested with a local server and the latest test servers and it worked fine.

The issue I got was only reproducible with dockers from a specific backend. Don't know why yet 🤔

[jesmrec]

After checking with servers deployed out of docker, everything works but the case in which an upload is cut off because lost connection