### What needs to get done - Raise an exception when the mountpoint does not exist in `Mount.mount()` - Catch exceptions when calling `Mount.umount()` in `_cleanup_chroot` to umount as many dirs as possible from the given list. ### Why it needs to get done If the mountpoint given to `Mount` does not exist, nothing is mounted. This may later break the command executed in the chroot. Detecting early that a target cannot be mounted should be easier to manage. When cleaning up the chroot, mounted dirs are unmounted in reverse order. A umounting failure is not caught, so if one umounting fails the current implementation does not try to umount the rest of the list. We should try to left the system in the most clean state so a umounting exception should be caught and raised only after attempting to umount every other mounted dirs.