Installer fails to "upgrade" a partial Tails installation
On "clone & upgrade", if the target Tails partition has some files remaining from a broken, or partially removed (e.g. after another failed upgrade) Tails installation, then
self.live.existing_live_os is false, and then the installer does not deletes the existing files before copying the new ones, and then it fails in similar ways to what is fixed by liveusb-creator's
In "clone & upgrade" mode, all files that are expected to exist in a Tails system partition should be deleted whenever they exist, regardless of what
self.live.existing_live_os thinks is in there.