Since they're not types common to all the services but belongs to the
configstore.
Next step will be to make them local to the configstore and not directly used by
other services since these types are also stored.
* client: always parse the json error message field and return its contents
* Use ErrBadRequest and ErrNotFound in every handler and command
* Gateway: by default pass underlying service error (configstore, runservice) to
client keeping the status code and message. In future, if some errors must be
masked, we should change the specific parts that need special handling.
* Always return a json message also on error. For internal errors return a
generic "internal server error" message to not leak the real internal error to
clients
* Return 201 Created on resource creation
* Return 204 No Content on resource deletion and other action with no json
output
Simone Gotti