When downloading consensus files, try other authorities when the first one fails

[cgrigis]

When running the script to download the consensus-related files, sometimes the download fails with the following error:

(...)
urllib.error.HTTPError: HTTP Error 503: Directory busy, try again later

(see e.g. https://github.com/c4dt/lightarti-rest/runs/4303347719?check_suite_focus=true)

To increase robustness, as there are 9 different authorities, we can try to download from another one in case of failure.