fix use of drupal_set_http_header() in RESTServer

The way that the REST server's error handling code uses drupal_add_http_header() isn't consistent with the way it's used in core and is probably incorrect. The attached patch fixes this (and incidentally makes logging easier).

For reference, here's an example from bootstrap.inc:
drupal_add_http_header('Status', '404 Not Found');
and from RESTServer.inc:
drupal_add_http_header('HTTP/1.0 204 No Content', $exception->getMessage());