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.
We shouldn't be encoding knowledge of the tag internals in other tests. If you change it, you break the tests.  This is why I suggested that names.NewActionTag should take a unit tag and a sequence.