Updates a hunt (it must be paused to be updated).

def UpdateHunt(hunt_id, client_limit=None, client_rate=None, duration=None):
  """Updates a hunt (it must be paused to be updated)."""

  hunt_obj = data_store.REL_DB.ReadHuntObject(hunt_id)
  if hunt_obj.hunt_state != hunt_obj.HuntState.PAUSED:
    raise OnlyPausedHuntCanBeModifiedError(hunt_obj)

  data_store.REL_DB.UpdateHuntObject(
      hunt_id,
      client_limit=client_limit,
      client_rate=client_rate,
      duration=duration)
  return data_store.REL_DB.ReadHuntObject(hunt_id)