fix(deisctl/units): short-circuit if we don't have an image

Using `;` instead of `&&` means the command won't exit if we
don't get an image.
fix(deisctl/units): don't fail units if ExecStopPost fails
Merge pull request #2079 from carmstrong/safe_stop_units

fix(deisctl/units): safe stop units; don't use `;` in units