I'm not sure how successful coulbe sending JavaScript errors to the server side be. It would be possible by firing another AJAX request (maybe using raw XmlHTTPRequest interface) but I'm not sure if it's not an overkill. The routine itself could generate errors so you have to take it into account.
The CallBackErrorJavascript receives an error from the WebForm_DoCallback (defined by ASP.NET) and it only gets errors from the AJAX calls. ASP.NET is not very smart here and it only throws these errors when there is a server-side exception generated during the AJAX callback. I guess it should be possible to catch these exceptions using ELMAH on the server side.