Getting the last error in a custom error page

When you provide a custom error handler page, it is useful to be able to log what caused the problem, so that you can fix it.
Normally, this is just a case of using

Exception lastError = Server.GetLastError();
if (lastError == null)
    {
    ErrorLog("An error occurred, but no error information is available");
    }
else
    {
    ErrorLog(lastError.ToString());
    }

This fails however, because the redirect to the error page loses the error information.
You can handle a Page Error event and pass the error through in a session variable, but that is a bit clunky, and means coding in your Master page and / or each actual page, plus the session variable itself.
Alternatively, there is a redirect mode as part of the custom error setup which redirects without re-loading. This preserves error information.
Change your customErrors declaration from:

<customErrors defaultRedirect="~/Whoops.aspx" mode="On"/>

To

<customErrors defaultRedirect="~/Whoops.aspx" mode="On" redirectMode="ResponseRewrite"/>

Then GetLastError will work fine.