Error handling Sarissa wrapper

In a previous post I've included a simple Sarissa wrapper that could be used to load an XML document (for example, XSLT stylesheet). If you want to perform more error checking and generate sensible errors, you could use these two functions:

• loadFromServer function requests object load. If you provide a callback parameter, the load is asynchronous, otherwise it waits for the load to complete and return the wrapper object.
• checkLoadStatus checks the various error conditions and throws appropriate errors.

var AWS = new Object();

AWS.loadFromServer = function (url,cb) {
  var lso = new Object() ;

  function loadHandler() {
    if (lso.request.readyState == 4) cb(lso); }

  lso.savedURL = url ;
  lso.request  = new XMLHttpRequest() ;
  lso.request.open("GET", lso.savedURL, cb ? true,false);
  if (cb) lso.request.onreadystatechange = loadHandler;
  lso.request.send(null);
  
  return lso;
}

AWS.checkLoadStatus = function (lso) {
  var url = lso.savedURL ; 
  var xr  = lso.request ;
  var msg;

  if (!url) throw("AWS.checkLoadStatus expects loadFromServer object") ;
  if (xr.status != 200) throw (url + " failed: "+xr.statusText);
  try {
    var de = xr.responseXML.documentElement;
    if (!de) msg = url + ": response is not a valid XML document";
  } catch(err) {
    msg = url + ": response is not a valid XML document (" + err + ")";
  }
  if (msg) throw(msg);
  return xr.responseXML;
}