Retry on HTTP 500

[infinite-turtles]

AWS S3 recommends retries on both 500 and 503 HTTP response status codes https://aws.amazon.com/premiumsupport/knowledge-center/http-5xx-errors-s3/

[tsutsu]

This probably shouldn't be a default behavior; most hosts will only emit 500 errors for logic errors that will take at least a manual redeploy of their infrastructure to fix, not transient operational errors that will be fixed by retrying.

What I would suggest is to add a flag (e.g. --retry-on-http-status) that lets you specify a list of additional HTTP status codes that should trigger a retry. People using aria2 with S3 could then set --retry-on-http-status=500.