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.