prototype.message is the default error message.

[tangxinfa]

Ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/message

Error.prototype.message is default message of instances, for example:

var NotFoundError = Error.extend("NotFoundError", 404);
NotFoundError.prototype.message = "Not Found";
var err = new NotFoundError();
console.log(err.toString());  //NotFoundError: 'Not Found'

It's better if we can specified default error message, for example:
var NotFoundError = Error.extend({name: "NotFoundError", message: "Not Found", code: 404});