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.

DEPENDS: #617, #651 
This case doesn't feel that special. What's so bad about the missing-hook error message? Shouldn't we just fix that error message, rather than replacing that one here?