Improve handling of removed/unavailable media during backup or restore

[t-m-w]

TODO: How does this affect e.g. WebDAV? That's a more likely scenario for this to be a legitimate problem than a USB drive being unplugged.

Backup

If a backup location becomes unavailable during the backup of user data (of apps), the notification may appear to keep going, especially for apps with nothing to back up. This looks very odd. Then, when it reaches an app with something to back up, it appears to hang.

Restore

If a backup location is unavailable or becomes unavailable during restoration, throw a user-visible error or complaint. Currently, for example, if a USB drive becomes unavailable before the restore-data phase, then when that phase starts, the restore process hangs on "System package manager", with the spinning progress thing next to it. Unclear if this will ever time out or not. (Edit: After about 30 minutes, there has been no timeout; it's still like this.)

Screenshot

If the restore has already started, however (progressed past the system package manager), and the media is removed (USB flash drive in this case), the remaining apps fail quickly, and an error is shown, so that case is fine, I think.