chore: add failover to alternative endpoint

[aanorbel]

Closes ooni/ooni.org#1958

[sdsantos]

Works well but:

1. Now we have a production API url as the debug fallback. It will upload, but with wrong anon credentials info, and the explorer dev URLs won't work for it. Either we have a debug fallback as well, or we should disable that for debug, to avoid some weird issues.
2. Eventually we should use the fallback on other requests too (out of the scope of this PR)

[aanorbel]

I think we can remove the prod URL fallback in favour of falling back to the dev API itself. This would ensure consistency of the environment untill the dev API has a fallback.