dialogDeviceMissing: Don't consider already mounted an error

This happens when creating a new backup configuration from an already mounted location via the remote location entry. It can also happen in any case when someone is browsing the backup location while a backup is being started.

I don't see much of a problem of ignoring this condition, as long as the path is available afterwards everything is fine. And if the path isn't available we will have a different error anyways.

Closes #182 (closed)