Magento – Preference is not working in Magento 2

I am trying to override ErrorHandler class using preference but it is not working.
di.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
   <preference for="Magento\Framework\App\ErrorHandler" type="Pulsestorm\HelloWorldMVVM\Model\ErrorHandler"/>

</config>    

ErrorHandler.php

<?php
namespace Pulsestorm\HelloWorldMVVM\Model;

class ErrorHandler extends \Magento\Framework\App\ErrorHandler{
    public function getHandler()
    { echo"hii"; exit;
        if (strpos($errorStr, 'DateTimeZone::__construct') !== false) {
            // there's no way to distinguish between caught system exceptions and warnings
            return false;
        }

        $errorNo = $errorNo & error_reporting();
        if ($errorNo == 0) {
            return false;
        }

        $msg = isset($this->errorPhrases[$errorNo]) ? $this->errorPhrases[$errorNo] : "Unknown error ({$errorNo})";
        $msg .= ": {$errorStr} in {$errorFile} on line {$errorLine}";
        throw new \Exception($msg);
    }
}    

I used echo exit to check preference is working or not. When any error or exception comes it doesn't echo and exit. Please let me know what I am doing wrong. Thank you in advance.