Make systemdUnitManager.Trigger{Start,Stop}() return errors, if their
underlying library functions from go-systemd return errors.
FakeUnitManager must be also updated according to these changes.

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 coreos#1216

In TestSystemdUnitFlow(), if Trigger{Start,Stop} returns an error,
then stop immediately.