[whatwg/fetch] Reject promise on Error Response (#627)

## The Issue
I find working with this API fairly different to reason about when calling `fetch` and receiving an error response from an external resource. Currently, I am implementing a feature that has the following flow:

- `fetch` a resource with `GET`
- if said resource doesn't yet exist, do a new `fetch` call with `POST`
- continue doing things.

Currently, if the GET fails with a `404`, the promise is _not rejected_ and I am unable to handle the error appropriately (by creating what does not already exist).

## Proposed Change
The API that I (and others on my team) imagine and would typically expect to work with is as follows:

```js
const getMyThing = () => {
  // Try to GET it.
  fetch("https://myendpoint.com/myThing/thingName")
    // resolves! Awesome, let's process it.
    .then(thing => thing.process())
    /* 
      Error? 
      It would be nice if the rejection contained the Response, with headers and status code.
    */
    .catch(error => {
      if (error.status === 404) {

        // create it and return its promise.
        return createMyThing("thingName")
      }

      // throw the error.
      throw new Error(error)
    })
}
```

If this issue makes sense and is considered, I'm happy to help describe, implement or test to the best of my ability.

cc @Angarsk8 as a fellow coder with this issue.

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