Move error status code to 500

[youngnick]

What would you like to be added:

In various places, we've documented that having no endpoints available should variously be 404 or 503. @thockin suggested in the v0.5.0 API Review that we should consider 502 instead.

This issue covers agreeing that's the case, and then implementing that change.

Why this is needed:
Need to get breaking behavior changes done before we release v0.5.0.

[mehabhalodiya]

@youngnick This needs to be done only in v1beta1 or in v1alpha2 also?
I caught 404 in v1beta1/httproute_types.go. Not sure, which other files to look at!

[mehabhalodiya]

I would like to take up this one.
/assign

[youngnick]

In both please, we will keep the v1alpha2 and v1beta1 in sync for this release.

[youngnick]

We agreed that:

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

@mikemorris is going to write out a list of the different cases, and check we haven't missed anything.

[dpasiukevich]

/assign