Fix EZP-21547: headers not sent for cached errors

Fixes https://jira.ez.no/browse/EZP-21547.

Maybe not the right way to do it, but it at least highlights what needs to be fixed.

1. Makes sure error headers are stored in cache data (in addition to the error code & message)
2. Sends those headers again when using the cached version

## Testing

Without the patch, if you go to http://ez/Media/ as anonymous user, you get a 401. If you reload, you will get a 200 OK.
With the patch, you will consistently get a 401.