File permission issues cause backups to stop being created

I encountered a situation, where backups never finish, leaving the `in_progress` directory always there. After analysing, the culprit seemed to be a specific file, which had `0000` permissions set. Changing the permissions allowed backups to work normally. Such situations should probably be handled more gracefully. While zero permissions look fishy, I can imagine other permissions related situations that would also cause failures. Like files created when running processes with elevated privileges (`sudo `).