throw Exception while callback == true

Hi.
I have serious problem with exception handling while callback == true
I found in MSDN http://msdn.microsoft.com/en-us/library/ms153106.aspx client error handling with clientErrorCallback parameter. But it only works while exception is throwed in RaiseCallbackEvent server function.
But i need throw exception in Page_Load and Begen_Request in global.asax and i have no idea how to check in client side, that this exception was rised. I started debug how callback internaly works in javascript and client get status 500. But when this status is set, clientCallback and clientErrorCallback not rise so i dont have the way how to.
I use ICallbackEventHandler and no ScriptManaget on Page.
Example of code is on http://msdn.microsoft.com/en-us/library/ms153106.aspx and only put:  ...