add `-f` switch to curl

[AvverbioPronome]

reason: https://community.letsencrypt.org/t/500-interval-server-error-in-options-ssl-nginx-conf-rate-limit/118286?u=9peppe

explanation: 22. HTTP page not retrieved. The requested url was not found or returned another error with the HTTP error code being 400 or above. This return code only appears if -f, --fail is used. https://ec.haxx.se/usingcurl/usingcurl-returns