Magento Whoops (https://github.com/filp/whoops) integration.

Mage_Core_Model_App.php

<?php
[...]
/**
 * Redeclare custom error handler
 * with Whoops support
 * @see https://github.com/filp/whoops
 *
 * @param   string $handler
 * @return  Mage_Core_Model_App
 */
public function setErrorHandler($handler)
{
    if(Mage::getIsDeveloperMode()) {
        $run = new Whoops\Run;
        $handler = new Whoops\Handler\PrettyPageHandler;
        $handler->addDataTable(
            'Magento', array(
                'Version' => Mage::getVersion()
            )
        );
        $run->pushHandler($handler);
        $run->register();
    }
    else {
        set_error_handler($handler);
    }

    return $this;
}
[...]