check if service is running or installed when uninstalling

[magynhard]

want proper messages when trying to uninstall not installed service

 - disable service 'my.service' ... Failed to execute operation: No such file or directory
done
 - delete service file '/lib/systemd/system/my.service' ... rake aborted!
Errno::ENOENT: No such file or directory @ unlink_internal - /lib/systemd/system/my.service