How do you handle errors from AJAX calls?

$.ajax({
   type: "POST",
   url: "MyUrl",
   data: "val1=test",
   success: function(result){
        // Do stuff
   }
 });

$.ajax({
   type: "POST",
   url: "MyUrl",
   data: "val1=test",
   success: function(result){
       if (result.error == "true")
       {
           alert("An error occurred: " & result.errorMessage);
       }
       else
       {
           // Do stuff
       }
   }
 });

$.ajaxSetup({
    error: function(xhr){
        alert('Request Status: ' + xhr.status + ' Status Text: ' + xhr.statusText + ' ' + xhr.responseText);
    }
});

header('HTTP/1.0 419 Custom Error');

$.ajax({
   type: "POST",
   url: "MyUrl",
   data: "val1=test",
   success: function(result){
        // Do stuff
   },
   error: function(request,status,errorThrown) {
        // There's been an error, do something with it!
        // Only use status and errorThrown.
        // Chances are request will not have anything in it.
   }
 });