Add a retry in case of 520 Cloudflare status

[steve-chavez]

Problem

Sometimes network error can happen due to Cloudflare network errors.

Proposal

Add retries whenever 520 status occur.

Notes

• This was previously tried on wip: automatic retries for transient failures #267
• There's also a gist here https://gist.github.com/thebengeu/8f564dbd7dfe660dda325fdfa1d06042

[soedirgo]

Adding a docs page on how to do retries is preferred here, there are too many variables in retry behavior (when to retry/which status codes, which retry algorithm to use) for us to be able to expose it as a stable interface.

[steve-chavez]

Ok, let's not do this now.