Allow caching error pages (404 etc.)

For sites that come under heavy load, e.g. DDoS, it's important to offload pages with an error status (404, etc.) to the CDN.

Currently Fastly module makes these pages uncacheable: drupal_add_http_header('Surrogate-Control', 'no-store');

It would be nice to have an option to turn off that no-store header so Fastly can fall back to the Cache-Control header (which the Drupal site might already be sending).