Java tutorial two: acknowledge only upon success

[dvdsk]

This would fix issue #254 by removing the try finally that caused the worker to acknowledge even if the doWork function ran into an exception as the finally branch of a try-finally is always run, even if an exception is raised in doWork.

Instead a comment is added explaining that an exception would cause the worker to crash and not ack

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[dvdsk]

If this is merged I will open a second PR in rabbitmq-website (code from tutorial was copied to https://github.com/rabbitmq/rabbitmq-website/blob/live/site/tutorials/tutorial-two-java.md)

[michaelklishin]

The current approach is not a mistake. Unconditional acknowledgement can be valid as well: not every delivery should be requeued. Of course, this begs the question "why don't you just use automatic acknowledgements then" and it's fair.

Perhaps the approach in this PR makes more sense in a tutorial but the current implementation has its merits as well.

[michaelklishin]

With this PR an exception would indeed result in no acknowledgement but it also would NOT result in delivery requeueing because the channel would not be closed and this PR does not requeue explicitly.

We should either leave this code as is with a comment or requeue explicitly.