A note to myself, of something I found on MSDN – for the ‘failed’ function on an asynchronous JavaScript call in the SharePoint Client Object Model:
function OnItemQueryFailed(sender, args)
{
alert('Error:\n' + args.get_message() + '\n' + args.get_stackTrace());
}
Note that these details are from the SP.ClientRequestFailedEventArgs class. There are other properties too.