How do I customize the error page

Question

Our IT department has adopted a policy that requires a standard look and feel for all error pages encountered in any web application. To this end I need to be able to modify the error page that shows up on the browser when an unexpected error is encountered or a validation error is sent to a new page.
How can I modify this page? Is there a template for it, and if so where?
Any help is greatly appreciated,
Adam Cumming

Answer 1

Hello Bhavin,
I have already started looking at this links (you copied twice Marc Lancaster's page, instead of Roel Hartmans's, which is actually : http://roelhartman.blogspot.com/2009/09/transform-ora-xxxx-messages-into.html)
I'm focused on Marc Lancaster's solution (Friendly Exception Messages), which is also interesting because it covers Ajax manipulation inside Apex (i'm using Apex 2.1).
But it seems that the apex.ajax.ondemand(...) line doesn't do anything to my application. Here is the code of this line :
var lRequest = new apex.ajax.ondemand('test', function(){     
                                                      var l_s = p.readyState;
                                                    if(l_s == 1||l_s == 2||l_s == 3){
                                        }  else if(l_s == 4){
                                        gReturn = p.responseText;
                                        $s('ajax-drop',gReturn); }
                                        else{return false;}
                                          );Because that's the first time I'm trying Ajax, I don't see what could be silly (I just copied the code from Marc's example).
Here is the application process, called 'test' :
begin
  htp.p('Hello !');
end;Thanks a lot.
Dovik