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.

DEPENDS: https://github.com/juju/juju/pull/651
I thought we were going to put these related fields into their own struct? *surely* nicer to have a single thing that is/is not nil, than to have a number of fields whose validity needs to be inferred from the nilness of one of them