As mentioned in the docs CakePHP ships with error renderers for both web and console environments. If however, you would like to replace the logic that renders errors you can create a custom error renderer class which then outputs json.
But you can also try something like GitHub - mixerapi/mixerapi: CakePHP API Plugin for REST + JSON API Development [READ-ONLY] which has
- Error Handling: Handles exception rendering in XML or JSON.
as you can see in the readme
But if you only need the exception rendering part you can also just use GitHub - mixerapi/exception-render: Handles rendering entity validation errors and other exceptions for your API [READ-ONLY]