Example "subclass" of Error object

HttpError.js

define(function () {
    var HttpError = function (msg, code) {
        var ex = new Error(msg || 'Unknown HTTP error');

        ex.code = code;
        ex.toString = function() {
            return 'Error: '
                + (this.code ? '' : '(' + this.code + ') ')
                + this.message;
        };

        return ex;
    };

    return HttpError;
});

usage.js

throw new HttpError('session timed out', 405);

/*
this has several advantages over `throw 'session timed out'`, including:

1. browsers will include a full stack trace
2. other data (e.g. status code) can be carried along

I had some issues with several browsers when trying to inherit from the 
native Error object.  That would be even better because then we could do
this:

if (ex instanceof HttpError) { ... }

*/