Installation fails without explanation on apt download errors

[seb128]

We had a few reports of installation failing which after reading the curtin log turned out to be due to apt failing to download packages. Ideally we would be robust to flaky internet but if not we should at least try a way to display a proper error dialog explaining the reason

[mwhudson]

Yeah, something like half of the error reports we get from subiquity boil down to this, so letting users know what's going on is a priority. I have a plan for how to do this in the server tui (https://asciinema.org/a/lqb9jrjUouRqxs1tMFsceZdGI) but aiui the desktop installer doesn't have a mirror screen that's shown by default so I don't quite know how it should be presented there.

[dbungert]

To make progress on this I've opened:

• https://code.launchpad.net/~dbungert/ubuntu-seeds/+git/ubuntu/+merge/410975
• https://code.launchpad.net/~dbungert/ubuntu-cdimage/canary-curtin/+merge/410976