4.3. Request Errors and Session Expiration Handling

RichFaces lets you redefine the standard handlers responsible for processing exceptions. We recommend defining your own JavaScript, which will be executed when exceptional situations occur.
Add the following code to web.xml: 
<context-param>
	<param-name>org.ajax4jsf.handleViewExpiredOnClient</param-name>
	<param-value>true</param-value>
</context-param>

4.3.1. Request Errors Handling

To execute your own code on the client in the event of an error during an AJAX request, you must redefine the standard A4J.AJAX.onError method like so: 
 A4J.AJAX.onError = function(req, status, message){
    window.alert("Custom onError handler "+message);
}
This function accepts the following as parameters:
req
a parameter string of a request that calls an error
status
the number of an error returned by the server
message
a default message for the given error
Therefore, you can create your own handler that is called when timeouts, internal server errors, etc. occur.