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

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
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