Using the Linux installer, behaviour is unexpected if user is upgrading and existing service not found

If you select an upgrade installation, but the existing service is not found (by checking for the presence of /etc/init.d/atlbitbucket or /etc/init.d/atlstash), then even if you select the option to "Install as a service" the option will be silently turned off, and the service will not be installed and the atlbitbucket user not created if upgrading from pre-4.0

In this scenario we should probably warn the user that they want to upgrade a service installation, but the existing service was not found, and if they elect to continue they will need to clean up the existing service manually as we will not be able to uninstall it for them.