docs: function(error, body){
    if (error < -1 || error > 4){ 
      throw "Invalid error code: " + error;
    }
    this.error = error;
    if (!Buffer.isBuffer(body)){
      throw "The body parameter must be a Buffer object.";
    }
    this.body = body;  // a Buffer object
    this.length = body.length + 2;  // body length + error field length
  }