Schedules a legacy AFF4 user notification.

def _NotifyLegacy(username, notification_type, message, object_reference):
  """Schedules a legacy AFF4 user notification."""

  try:
    with aff4.FACTORY.Open(
        aff4.ROOT_URN.Add("users").Add(username),
        aff4_type=aff4_users.GRRUser,
        mode="rw") as fd:

      args = _MapLegacyArgs(notification_type, message, object_reference)
      args[0] += ":%s" % notification_type
      fd.Notify(*args)

  except aff4.InstantiationError:
    logging.error("Trying to notify non-existent user: %s", username)