Add fetch response and error utils

[BarryThePenguin]

Transitioning from ember-ajax to ember-fetch and noticed there weren't any error detection helpers

This PR adds similar helpers, mostly a copy/paste. Main motivation is to aid in transitioning between the two.

import {
  isNotFoundResponse,
  isForbiddenResponse
} from 'ember-fetch/errors';

I looked through npm briefly to see if something similar already existed, but couldn't find anything. If we want to move forward with this, I can add some documentation to the readme.

[xg-wang]

I think these are helpful to make people using fetch api easier, but there're some differences when migrating from Ajax to Fetch, I did a quick check on our polyfills (https://github.com/mo/abortcontroller-polyfill and https://github.com/github/fetch) my comments about response type are also true. Can you make the code changes and explicitly call out in Readme doc?

[BarryThePenguin]

Thanks for the review. It was very helpful. I learned a lot reading the fetch and related specs

I've added a section to the readme too