FIX: ErrorPage generating empty responses for 403/401 requests

Previously by setting the response status code inside the action, this prevented response bodies from being included due to 403/401 being matched by SS_HTTPResponse::isFinished() (which stops popular

I assume SS_HTTPResponse::isFinished() is valid for the permission error use case (and I would be hesitant to change it) so this simply moves the declaration of the response status code till after the parent has populated the body of the response.
Merge pull request #898 from dnadesign/errorpagepermissions

FIX: ErrorPage generating empty responses for 403/401 requests