Re: [fetch] Why doesn't `fetch` reject if 400 ≤ status < 600? (#18)

I also ran into the same issue - 404s didn't cause errors, thus I wasn't able to catch() the issue and handle it. https://github.com/github/fetch/issues/155

"there's nothing exceptional about 4xx [...] errors on the web."

Fetching the file failed - that's a failure, no matter whether it's called exception or error, IMHO.

I venture to guess that there will be many more reports of people being surprised that fetch doesn't throw when there's a 404. And what's worse, there probably will be many 404s going unhandled, because many will expect fetch to throw on 404.

fetch (at least in FF https://bug1126483.bugzilla.mozilla.org/attachment.cgi?id=8555602 ) reports "response.ok" for status "204 No Content". For my current app that's not what I want, so I'll simply throw an error on any non-200. (When the status is 200 the content could still be zilch, but if it's certain that there's no content, I shouldn't resolve the promise I'm returning.)

```
function fetchStatusHandler(response) {
  if (response.status === 200) {
    return response;
  } else {
    throw new Error(response.statusText);
  }
}
'''
and (in my case inside a larger Promise block):
```
fetch(url)
  .then(
    fetchStatusHandler
  ).then(function(response) {
    return response.blob();
  }).then(function(blob) {
    resolve(blob.size);
  }).catch(function(error) {
    reject(error);
  });
```

For the spec:

In order to prevent the user (who might expect fetch to throw on 404) from accidentally ignoring 404s etc, perhaps raise an error if there's no status code handler supplied by the user. (eg a required arg {statusHandler: func} .)

I also like the solution proposed earlier by Anne, eg with a different name:
```
fetch("...", {throwUnlessStatusIs: "20x"})
```
It should be a required arg so that the user knowingly accepts or rejects eg 404s.

---
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