"pmbootstrap export" without building system image: 'Namespace' object has no attribute 'add'

Created by: ollieparanoid

Reported by Bloo on IRC. As I see it now, it shouldn't even try to make the system partition in case it is missing (because then the --add parameter would only work the first time). Instead it makes sense to me to simply not export it, if it is missing (and maybe write a notice on how to generate it).