What is the right way to get the response text to a failed Store operation?

So I'm saving a record to a store with an AJAX proxy, and I'm trying to get hold of the error message when it fails. Doesn't seem like this should be the way to do it, but I don't see another way.

Code:

    record.save({
        callback: function(records, operation, success) {
            maskedElement.showMask(false);
            if(!success && operation.error.status == 410) {
                 msg = operation.request.callback.arguments[2].responseText;  // This seems WRONG
            } else {
                callback && callback();
            }
        }

What is the right way to get the response text to a failed Store operation?