MINOR : Tolerate GroupIdNotFoundException in consumer when leaving a group.

[ShivsundarR]

What

• Currently if a consumer/share-consumer calls close() before it has joined a group, then the heartbeat on close will be sent with epoch = -1 and the broker would return "GroupIdNotFoundException".
• This was causing couple of tests in ShareConsumerTest to be flaky if the heartbeat to join the group was sent with epoch = -1.
• Since this can occur in real scenarios as well, it would be better to tolerate this exception while we are leaving the group so that the consumer can close cleanly.

[AndrewJSchofield]

@ShivsundarR I still get a failure of testSubscribeOnInvalidTopicThrowsInvalidTopicException even with this patch.

[ShivsundarR]

Thanks @AndrewJSchofield, I was using the incorrect state for the check, it should actually be MemberState.UNSUBSCRIBED, since we move from MemberState.LEAVING to MemberState.UNSUBSCRIBED after sending a leave request.
I have made this change now, and ran the test locally for 100 times and did not get a flake.