fix: return 502 for errors caused by particular upstream configuration

[adubovik]

Logical follow-up of #378

DIAL Core upstream is identified by its endpoint and an optional key.
This information is passed to the adapter in request headers X-UPSTREAM-ENDPOINT and X-UPSTREAM-KEY.

Whenever the adapter encounters an upstream endpoint error linked to the endpoint identity (those upstream-specific headers), it should return 502 to signal to DIAL Core to try another endpoint.

Status code translation examples

Status code from upstream	Status code from adapter	Comment
400	400	Since an invalid request is very likely to trigger the same 400 error, retries with other upstreams won't help.
500	502	The upstream encountered an internal error specific to that upstream; retries may help.
401/403 (X-UPSTREAM-KEY header present)	502	The upstream key was likely misconfigured, so it makes sense to try another upstream.
401/403 (X-UPSTREAM-KEY header missing)	401/403	The adapter tries to authenticate via Azure AD and fails. Retries won't help.

[adubovik]

/deploy-review

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