Uniter Action(id string) API client/server

This commit adds the ability for Uniter clients to query Actions from
state.State by id.  The response includes a server error message if the
Action is not present on the state server.
Just working off apiserver/uniter/uniter.go Relation on that one.  If getOneRelation fails, it composes the new struct using the error and returns it to the requester, presumably so the requester will know what's wrong with the call.