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.
While at it, we might also consider adding similar try-catch blocks to two other cases (UPSERT, DELETE) of the surrounding switch statement.