feat(PubSub): Add SubscriberErrorListener sample

[cy-yun]

https://buganizer.corp.google.com/issues/416330649

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• pubsub/api/src/subscriber_error_listener.php:27, tag pubsub_subscriber_error_listener

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment