silverstripe-cms/code/Logging/ErrorPageErrorFormatter.php

<?php

namespace SilverStripe\CMS\Logging;

use SilverStripe\Framework\Logging\DebugViewFriendlyErrorFormatter;
use SilverStripe\CMS\Model\ErrorPage;

/**
 * Provides {@see ErrorPage}-gnostic error handling
 */
class ErrorPageErrorFormatter extends DebugViewFriendlyErrorFormatter {

	public function output($statusCode) {
		// Ajax content is plain-text only
		if(\Director::is_ajax()) {
			return $this->getTitle();
		}

		// Determine if cached ErrorPage content is available
		$content = ErrorPage::get_content_for_errorcode($statusCode);
		if($content) {
			return $content;
		}

		// Fallback to default output
		return parent::output($statusCode);
	}
}