I created a subclass of PHP's Exception class (which I'll call MyException) because I needed to add an additional property that contains a more user-friendly message for the user. So aside from the usual properties of the Exception class (i.e., $message, $code, ...), I added a property called $userMessage. I ran into no problems with this, and it works very well within PHP.
But when I get to the final moment when I throw a MyException within PHP for .ajax()'s .fail() handler to receive, how do I access this custom property (i.e., $userMessage) from the jqXHR object? It wasn't the 'statusText', not the 'responseText' property either.