How is a JSON error generated when using Jetty Servlet?

Question

Jetty

Interested in such a question - at what stage (filter) is an error generated in JSON format?
I have a task to add a custom attribute to this JSON. I need to solve this problem using filters.
I have two follow-up questions:
1. In which of the filters this error JSON is generated
2. How to insert your filter after processing the filter from (1 point).
Thank you for your time!