When ran locally, the backup ends ends up in Tomcat's bin folder if the path is relative. On OS X the backup will be created but the user will not find it, on Linux the permissions will be missing.

So it would be good to enforce an absolute path, and to also check for write permissions while we're at it.

The check must be done in BackupExecutor and replicated in BackupPresenter (because the scheduled task configuration of the Backup app doesn't hit the executor directly).