The current spec defines that addModule() rejects a promise with "AbortError" DOMException in any case:

2. Reject promise with an "AbortError" DOMException.
   (see https://drafts.css-houdini.org/worklets/#fetch-and-invoke-a-worklet-script)

This obfuscates the reason why addModule() failed and makes it hard to debug. It would be nice to reject it with a more specific exception like "NetworkError" and "SyntaxError".