errorPage method should return a 410 Gone

[JamesDPC]

Hi folks

The errorPage method currently returns a '200 OK' when called and renders a Silverstripe logo with an ipad by default:

    /**
     * @return DBHTMLText
     */
    protected function errorPage()
    {
        Requirements::css('silverstripe/sharedraftcontent: client/dist/styles/bundle-frontend.css');
        return $this->renderWith('ShareDraftContentError');
    }

When a token expires or is not found, the URL should return a 410 HTTP response and let the application handle how it renders that status code (eg. default web server error page).

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/410

The 410 status code would instruct HTTP clients to not request that URL again.

Cheers
James