ActionFinish called after Action completion

The worker now calls ActionComplete or ActionFailed on state, depending
on the success state of the Action and any errors.

The hook is set to an error state if the Action hook is missing, or if
other errors take place during hook execution.  If the Action hook sets
a fail state, the Hook will not error, rather the Action will fail.

DEPENDS: https://github.com/juju/juju/pull/651
so *that's* why we check action failed. That feels kinda evil to me -- we have a RunAction method that you have to call, to make it *not* run the action but still finalize the context?