Function log_failure 

fn log_failure(
    dest: &Destination,
    error: &Error,
    statuses: &HashMap<Destination, TransactionStatus>,
)

Log a failed transaction, at warn when a content rejection starts a destination’s streak.

A content rejection records no peer backoff and its batch replays unchanged, so without the warning a stuck destination is silent at default levels. Call it after fail_status, whose advanced status it reads.