[whatwg/fetch] Using "no-cors" to distinguish between a network error and a request blocked by CORS (#1123)

I am not sure if this is the correct place for this but it currently is possible to distinguish between a request blocked by CORS and a request that failed to a network error.

By submitting two requests, one with mode: "cors" and the other with mode: "no-cors"

```
req. 1 success  | req. 2 success  | result
       yes           |          N/A        | CORS is allowed
       no            |          yes         | CORS is disallowed
       no            |          no          | Network error
```

I'm seeing several issues asking for a way to distinguish CORS blocked requests from requests that have failed due to an actual network error and security has come up several times as a reason as to why requests blocked by CORS shouldn't be distinguishable so this seems rather inconsistant with how mode: "cors" requests operate.

<details><summary>Sample code</summary>
<pre>
async function testCors(url) {
    //First try to make a request using mode: "cors"
    let failed = false;
    try {
        await fetch(new Request(url, {mode: "cors"}));
    } catch {
        failed = true;
    }
    //If the request succeeds, then the page isn't blocked by CORS
    if(!failed) {
        console.log(url + " is reachable and wasn't blocked by CORS");
        return url + " is reachable and wasn't blocked by CORS"
    }
    //At this point the first request has failed, now we try again but without CORS
    let failed2 = false;
    try {
        await fetch(new Request(url, {mode: "no-cors"}));
    } catch {
        failed2 = true;
    }
    if(failed2) {
        //If we still can't make a request, then it truly is unreachable
        console.log(url + " is unreachable");
        return url + " is unreachable";
    } else {
        //But if it does work, it confirms that the host exists and it is blocking our requests because of CORS
        console.log(url + " was blocked due by CORS but is still reachable");
        return url + " was blocked due by CORS but is still reachable";
    }
}
</pre>
</details>

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