agent: unload unit only when TriggerStop() runs successfully
agent: unload unit only when TriggerStop() runs successfully

In Agent.unloadUnit(), if systemdUnitManager.TriggerStop() returns
any error, do not unload systemd units. Otherwise the unit could get
into a state where the unit cannot be stopped via fleet, because the
unit file was already removed.

Fixes https://github.com/coreos/fleet/issues/1216

endocode/fleet,dalbani/fleet,coreos/fleet,dongsupark/fleet,dongsupark/fleet,giantswarm/fleet,coreos/fleet,dalbani/fleet,endocode/fleet,jonboulle/fleet,giantswarm/fleet,jonboulle/fleet,giantswarm/fleet,endocode/fleet,dalbani/fleet,dongsupark/fleet,jonboulle/fleet,coreos/fleet