How to trigger fetch(request).catch() block for testing?

As a novice javascript programmer I found the example code below

return fetch(request).catch(err=>err)

For testing I tried to trigger the catch clause by making bad requests (malformed url, non-existent domain, htp instead of http etc) but all seemed to just return a 4XX or 5XX http status code instead. (ie the catch() was never invoked)

The documentation suggests that fetch only fails for “network errors” but I’m not sure what that means.
Is it possible that “fetch()” ever fails within the webworker environment? If so how can I simulate that failure for testing?