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 seems to me like a fail state could be something reversible; for example, if a web API is busy, the Action might poll it until it receives a good response, at which point the error could be reset, or for 5 minutes.  If the 5 minutes are up before the failure is cleared, the final result will be a fail; otherwise, it seems like it could be cleared.