Just comment the following line in your servlet and evrything will start working.
PrintWriter out = res.getWriter();
In theory you should not have get any error becuase forward method of requestdeispatcher throws exception when output in response has been committed and you are trying to forward the request to something else ( in your case JSP ) . Since you have just obtained the writer for response should not result in error , because you haven't writen anything to the writer yet. This problem is evident in your error message. So just remove this line or comment it out.