DocumentMissingException is also thrown on update retries

- Closes #6355.
- Contains the bugfix and an integration test demonstrating the bug and validating the fix.
- The fix in TransportUpdateAction is a simple try-catch block calling the appropriate failure handler. The fix prevents DocumentMissingExceptions from ending up uncaught and thus makes sure that an HTTP response is actually sent to the client instead of not sending any at all.
You are right. Seems I was overly cautious when turning it into an ElasticsearchIntegrationTest. But as long as I only use my own internal engine implementation within my test indexes, everything is fine. Going to remove the annotation line completely.