ASP.NET UpdatePanel Timeout And 500 Errors With Custom Errors?

I have a .net application that uses customerrors web.config module to display meaningful messages for errors. It works without any issues for 500 errors/exceptions caused by non-ajax and ajax components (updatepanel). However, in a scenario where updatepanel's asynchronous request times out, there is no error raised at all. I was able to see the timeout in firebug and come up with a solution that would at least display the error message as an alert and then redirect the user to the 500 error page using javascript but it's not quite doing what the rest of the application does in case of an unhandled errors like these. I basically just want everything to go through "LogEvent" mechanism so based on the severity of the error, it does the necessary work.This 500 error page doesn't have anything in the Server.GetLastError() for these timeout scenarios. Is this an expected behaviour? Can it be changed so I do have access to these timeouts in Server.GetLastError() OR maybe just run this error through "LogEvent" mechanism? Is there a better/more graceful way to handle this issue?

Below is my code to give you an idea, not exactly what I have in my application but pretty close.

Web.Config
<customErrors mode="On" defaultRedirect="~/Errors/ErrorUnknown.aspx" redirectMode="ResponseRewrite">
<error statusCode="500" redirect="~/Errors/Error500.aspx" />
</customErrors>

[code]....