storage_stats: Actually check the HTTP status code

By default reqwest does not return an actual error for non-success HTTP
status codes like HTTP 404.

This lead to parser failures when trying to read the HTML error message
from a 404 reply as a deb822 document.

Use `Response::error_for_status()` to actually check for successful
status codes.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>