Nakadi doesn't properly report errors when events cannot be consumed from a subscription

[juhovuori]

A low-traffic event-type on Nakadi may cause subscription to enter a state where trying to read events from it returns to HTTP 200, and immediate closing of TCP connection. Apparently an error message is loogged about a stale subscription, but client has no way of telling what happened. A proper error message would make it possible to handle this automatically in client.

From @ferbncode in google chat: "For https://nakadi-ui-staging.aruha-test.zalan.do/#subscriptions/205ee6e5-acf2-4c1b-aa3d-d3ff1217e575, I see this in the logs org.zalando.nakadi.exceptions.runtime.InvalidCursorException: offset 00000000000000009 for partition 5 is unavailable. This might have happened because the unconsumed events of the subscription event type passed the retention period while the subscription stayed at the old offset. I guess Nakadi should not silently log and not return with 200."