First you have to make sure you can navigate to the error controller with your browser via http://localhost/error/error or http://localhost/default/error/error

If you get an "Object not found" error than you have to check your Apache httpd.conf and comment out:

# Multi-language error messages
#Include conf/extra/httpd-multilang-errordoc.conf

because of this directive Apache maps /error to path_to_your_apache_installation/error folder, and not through the .htaccess file to index.php

After you are sure that you ErrorController is working (you also need a scripts/error/error.phtml file), and it' accessible just make sure that you haven't set $frontController->throwExceptions(true) on your Front Controller, because it disables the ErrorHandler controller plugin.

p.s.
You don't need to register Zend_Controller_Plugin_ErrorHandler () with the Front controller using something like $frontController->registerPlugin (new Zend_Controller_Plugin_ErrorHandler ()); because the Zend_Controller_Plugin_ErrorHandler () is enabled by default.

Hope this helps...