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.
This is a little surprising when the normal idiom is `_, err := foo(); if err != nil { /* error handling */}`
Also, the `if err == nil { /* foo */ }`  approach increases nesting, which isn't ideal.