Bad UX when trying to import an old backup from a file

[Hocuri]

By increasing DCBACKUP_VERSION, we can make sure that an error will be shown when the user tries to import a new backup into and old Delta Chat version.

This works very nicely for Add-second-device, but for import-file, there are two problems:

1. Even though an error message is shown, the profile is still imported, and accessible via the account switcher.
   Instead, the profile should be deleted if DCBACKUP_VERSION is too new.
2. The error message shown is cryptic, and not localized:

Add-second-device	Import-file

[Septias]

Can you provide an outdated backup for testing?

[Septias]

1. Even though an error message is shown, the profile is still imported, and accessible via the account switcher.
   Instead, the profile should be deleted if DCBACKUP_VERSION is too new.

From what I saw, import_backup merely attaches the database from the backup file and not more. The frontend handles account creation (at least in the desktop case) and then imports a backup, so I see the responsibility to remove that broken account from the list on the frontend. One way to remove it in core would probably be to call Accounts::remove_account, but the context does not know about the account manager.

[Hocuri]

Can you provide an outdated backup for testing?

We recently increased DCBACKUP_VERSION: #6929. So, just export a backup with current main, and try to import it into a released version of DC