action-set: a jujuc command for Actions to send values back to state

action-set permits action hooks to define an arbitrary map of params
which will be returned to the state server at the end of the Action,
or to specify --failed with an optional failure message.
I don't think we ever want to know this on this interface, do we? The context will want to report it up, but I don't see why the hook commands need to know it.