As is, each entry in "error_categories" in error_codes.yml is a simple string value. It should be possible for these to instead be like so:

error_categories:
    - ShutdownError
    - {name: "CancelationError", subcategories: ["ShutdownError"],}

Each category should be a superset that consists of the codes for all of its subcategories as well as each code that lists it as a category. For instance, CancelationError would list ShutdownError as a subcategory.