Move error status code from 404 to 500

[dpasiukevich]

• If there are no matching routes, a 404 should be generated
• If there are matching routes but no/missing backends/endpoints to
  route to, a 500 (specifically) should be generated.

What type of PR is this?

/kind api-change

What this PR does / why we need it:
Update status code requirements to return 500 if there are no backends for the route instead of 404

Which issue(s) this PR fixes:

Fixes #1200

Does this PR introduce a user-facing change?:

The Gateway API now requires to return http 500 status code if there are no backends for existing route

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[k8s-ci-robot]

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[robscott]

/ok-to-test

[robscott]

Thanks @dpasiukevich!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dpasiukevich, robscott

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[mikemorris]

Should this have been changed to 500 too?

[dpasiukevich]

I agree, seems we should update here as well.
@robscott should I make the PR to change this status code 503 -> 500 as well?

[dpasiukevich]

Added the PR for that: #1210

[mikemorris]

I took a brief look at the existing conformance tests and I think the expected 404 status codes are all still correct (no followup changes needed), as they reflect unmatched routes - I didn't notice any tests for matched routes with no valid backends yet.

[dpasiukevich]

I could create the test. Is it needed to be done before the v0.5.0 release?

[robscott]

@dpasiukevich I don't think this is required for v0.5.0 release, but we should add a conformance test for it sometime soon. As I was writing up an issue for that test I ran into #1211, so may be a bit until we figure out exactly what that test should look like.