When we block a Worker or SharedWorker via CSP, we should fire the worker's `onerror` event rather than throwing in the constructor. That is, the following test should pass:

```
var w = new Worker(url);
w.onmessage = t.unreached_func("Ping should not be sent.");
Promise.all([
  waitUntilCSPEventForURL(t, reportedURL)
    .then(t.step_func(e => {
      assert_equals(e.blockedURI, reportedURL);
      assert_equals(e.violatedDirective, "worker-src");
      assert_equals(e.effectiveDirective, "worker-src");
    })),
  waitUntilEvent(w, "error")
]).then(t.step_func_done());
```

Currently, we throw, which is unfortunate.

Fixing dedicated workers is trivial. Fixing SharedWorkers is a nightmare.