code: function getExistingMessage(svc, name) {
    var msg = svc.message(name);
    if (!msg) {
      throw new Error(f('unknown message: %s', name));
    }
    return msg;
  }
docs: Get a message, asserting that it exists.
  
  @param svc {Service} The protocol to look into.
  @param name {String} The message's name.