MSI - product can become un-uninstallable

If certain files get deleted from the filesystem, most notably the python-based installer-util.exe, both Uninstall and Repair will fail and rollback.

Certainly Repair should work to restore the missing files - need to see whether we can prevent those Custom Actions from firing off on a Repair operation.

If Repair can be made to work, then we should leave Uninstall unchanged - we would want Uninstall to fail if it is unable to make the var backup. Ideally it would show a dialog suggesting the user Repair the installation and then try Uninstall again, but I don't know if that's possible.

However if Repair cannot be fixed to prevent these Custom Actions from being attempted, then we should make it so that ALL Custom Actions ignore failure, so that the product cannot get into this zombie state.