docs(samples): Optimistic subscribe sample

[michaelpri10]

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[snippet-bot]

Here is the summary of changes.

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• samples/snippets/src/main/java/pubsub/OptimisticSubscribeExample.java:19, tag pubsub_optimistic_subscribe

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[michaelpri10]

/gcbrun

[michaelpri10]

@kamalaboulhosn Quick bump of this PR review when you have the chance. Thanks!

[michaelpri10]

/gcbrun

[michaelpri10]

@kamalaboulhosn I've added a small change here to also catch the IllegalStateException and ignore it if the underlying failure is due to a NotFoundException. The exception being thrown was causing the tests to fail.