When CDX backend is unavailable, PyWB 200 status code instead of 503 status code

[ptrourke]

Describe the bug

When a CDX backend (such as OutbackCDX) is unavailable, PyWB returns a 200 status code rather than a 503 status code. As a consequence, our caches are caching empty results, and our load balancer is returning results from PyWB backends that are not healthy.

Steps to reproduce the bug

With a PyWB server that is using OutbackCDX as a backend, stop the OutbackCDX service.

Expected behavior

PyWB should return a 503, indicating that the service is unavailable.

Environment

• OS: Linux
• Browser: any

Additional context

We're seeing inconsistent results that we think may be due to caching of empty responses. This is probably the same issue as #627 and #843 .

[ptrourke]

Krisztina Thatcher (also of LC) is tracking this issue as well.