Returns a single access event by its event_id, plus the error logs the
same request produced.

event_id comes from the /access/events list. A raw req_id is not a
valid key — it is producer-generated and collides with unrelated requests
— and yields 400.

has_errors is tri-state: true/false answer the question, and null
means the error lookup did not complete. null is not "no errors".