The POST /os-server-external-events API had the following confusing
behavior:
With multiple events in the payload, if *some* (but not all) were
dropped, the HTTP response was 207, with per-event 4xx error codes in
the payload. But if *all* of the events were dropped, the overall HTTP
response was 404 with no payload. Thus, especially for consumers sending
only one event at a time, it was impossible to distinguish e.g. "you
tried to send an event for a nonexistent instance" from "the instance
you specified hasn't landed on a host yet".
This fix gets rid of that sweeping 404 condition, so if *any* subset of
the events are dropped (including *all* of them), the HTTP response will
always be 207, and the payload will always contain granular per-event
error codes.
This effectively means the API can no longer return 404, ever.
Closes-Bug: #1855752
Change-Id: Ibad1b51e2cf50d00102295039b6e82bc00bec058
e6f7425444