Consider not proxying 5xx status responses from upstream

Proxying 5xx status responses from upstream interferes with monitoring tools that use this status code to detect internal errors in a service. While we probably can't avoid proxying 4xx statuses because client-side code may rely on it, we probably can get away with substituting 5xx status responses with an alternate value. This would allow us to rely on 5xx statuses to detect the rate of internal errors for example.

See Slack discussion: https://hypothes-is.slack.com/archives/CR3E3S7K8/p1612979225052500?thread_ts=1612975098.050300&cid=CR3E3S7K8