PubSub: Propagate RetryError in PublisherClient.publish

[relud]

in google.cloud.pubsub_v1.publisher._batch.thread.Batch self._client.api.publish(self._topic, self._messages) may raise google.api_core.exceptions.RetryError.

if Batch._commit does not catch an error then Batch._futures may never complete.

google.api_core.exceptions.RetryError and google.api_core.exceptions.GoogleAPICallError are the only inheritors of google.api_core.exceptions.GoogleAPIError, so that seemed like the appropriate error to catch.

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[tseaver]

Seems like having a new unit test which raises RetryError from publish would be the right way to ensure that we don't regress this fix. Copy the api error test and have the error be a RetryError.

[tseaver]

@relud Thanks for following up with a test.