This code fetches a worker script, and dispatches an error event if the fetch is blocked for any reason. However, a fetch can be successful while still returning a response that is unusable (such as a 404 response), and we need to check that we can actually use the response before trying to execute it as a script.

Per https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-classic-worker-script, we need to implement step 2.2, and it should trigger step 1 of https://html.spec.whatwg.org/multipage/workers.html#worker-processing-model:concept-script-error-to-rethrow . We will need to extract the error implementation from the current code (linked above) so it's shared by both cases.

Fixing this should also fix #22991, which is an extremely frequent intermittent test failure. Try running it in a debug build before and after making this change to see the difference (./mach test-wpt /workers/constructors/Worker/Worker-constructor.html --no-pause-after-test).