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.
Yeah, catching Throwable might be even better to handle other cases as well. I was restricting myself to fix just the error situation I observed :-)