ActionComplete / ActionFailed 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.
It is too subtle to distinguish between a zero value action tag and an action tag that was set to this value delibrerately.

I suggest either, change ctx.actionTag to be a `names.Tag`, or a `*names.ActionTag`, then you can compare that value with `nil` to see if it is unset.