The ASP.NET AJAX failure callback takes a single parameter. From MSDN, a sample failure callback would look like this:
var stackTrace = error.get_stackTrace();
var message = error.get_message();
var statusCode = error.get_statusCode();
var exceptionType = error.get_exceptionType();
var timedout = error.get_timedOut();
// Display the error.
var RsltElem =
"Stack Trace: " + stackTrace + "<br/>" +
"Service Error: " + message + "<br/>" +
"Status Code: " + statusCode + "<br/>" +
"Exception Type: " + exceptionType + "<br/>" +
"Timedout: " + timedout;
wcfFailCallback function takes an
error parameter, which has a number of properties that provide you information about what failed.
The full article on MSDN is here. It goes into some decent amount of details about how to hook up WCF services to ASP.NET AJAX clients.
I hope this helps!! If there are other questions or if I didn't fully understand your question, let me know and I'll update my answer accordingly.