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.
It's simply returning true if ActionSetFailed has been used to set a non-nil ctx.actionErr.  Therefore, ActionResults tells us not only the results, but the fail state.