Revert unnecessary file_download() conversion from drupal_access_denied()/drupal_not_found() to throwing exceptions

This branch changes file_download() from calling the drupal_access_denied()/drupal_not_found() functions to throwing the Symfony exceptions directly. However, it does not change the corresponding PHPDoc to match, and there's follow-up issues to sweep all of core to do this (#1587850: Replace drupal_not_found() with throw NotFoundHttpException, #1591604: Replace drupal_access_denied() with throw AccessDeniedHttpException), so I suggest removing this outlier from the initial kernel patch. Here's the revert patch to do so.