updatectl: Show a helpful error if an update is partially downloaded

updatectl: Show a helpful error if an update is partially downloaded

If an update is partially downloaded and the user tries to update again,
`updatectl` can’t currently do anything (it doesn’t yet support resuming
downloads). At the moment, though, it’ll return success as if the system
was up to date, even though it isn’t up to date.

Instead, print a more helpful error message telling the user to try
vacuuming the partial version and trying again.

I decided not to make it automatically vacuum the partial version, as
that seems like a way to get into a nasty retry loop if, for example,
the checksum provided by the server doesn’t match that of the downloaded
file (which is one way to trigger this code path).

Add an integration test which simulates this failure by corrupting the
`SHA256SUMS` file, trying to download an update, and then working
through the recovery steps.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Fixes: https://github.com/systemd/systemd/issues/41502