Use better default error page settings

The default settings for the error pages are pretty poor. They aren't proper sentences.

I think the default should be as similar as possible to the drupal core ones found in drupal_deliver_html_page().

For example:

403 page title: Access denied
403 page content: You are not authorized to access this page.
404 page title: Page not found
404 page content: The requested page could not be found.