add multiple window.onload and onerror events

events.js

(function(){
	function addWindowEvent (event, fn) {
		var old = window[event];
		if (typeof old !== 'function') {
			window[event] = fn;
			return;
		}

		window[event] = function () {
			old.apply(window, arguments);
			fn.apply(window, arguments);
		}
	}

	window.addOnLoad = function (fn) {
		addWindowEvent('onload', fn)
	}

	window.addOnError = function (fn) {
		addWindowEvent('onerror', fn)
	}

})();