The status of the server response (200, 404, ...) in a synchronous page load?

On the server side you know the status. Pass it to JavaScript.
On PHP:
echo 'var status='.$status.';';

If you have received a response from the server 200, 404,... in primary synchronous load, the javascript code could not get to the client browser and cannot be executed.
If it is necessary, pay attention to the scripts embedded in the browser, but where is the guarantee that this plugin will set all your visitors?

It is possible using the XMLHttpRequest to request the same page again.
In General, what a strange task, I want to answer that you should not be required :-)

httpRequest.status — https://developer.mozilla.org/en/AJAX/Getting_Started.