action-fail: a jujuc command for Actions to set an error state

action-fail permits an Action to set an error value which will be returned to
the stateserver upon completion of the Action.  This does not prevent
action-set from adding more values to the results map.
if we're embedding that "action-requested-u/0_a_0" in an error string we need to be prepared for people to see it. Don't we have the action UUID available? I think the hook context ought to be setting that in the env: JUJU_ACTION_NAME, JUJU_ACTION_UUID, possibly more?