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.
Hmmm.  How would an actionParams (i.e. action arguments) fit in with the results?  They seem like two different pieces.  I think the Results and Err probably do belong together.  I'll stick that in as well.