KIP-400 — Improve exit status in case of errors in ConsoleProducer

Discussion Producer

Adds a `--exit-on-producer-error` flag to `kafka-console-producer.sh` and changes the default async-mode behavior to track send callback failures and exit with a non-zero status code after all records are sent if any failed. In async mode, the existing `ConsoleProducer` swallows send errors and always exits with status 0, making it impossible for shell scripts to detect data-loss failures.

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