[whatwg/fetch] fetch keepAlive inflightKeepaliveBytes errors cannot be handled (Issue #1816)

DerGernTod created an issue (whatwg/fetch#1816)

### What is the issue with the Fetch Standard?

The [spec](https://fetch.spec.whatwg.org/#http-network-or-cache-fetch) is very clear about what should happen when `inflightKeepaliveBytes` exceed the limit of 64kib:
> If the sum of contentLength and inflightKeepaliveBytes is greater than 64 kibibytes, then return a [network error](https://fetch.spec.whatwg.org/#concept-network-error).

This means that the user can't differentiate between requests that failed because the `inflightKeepaliveBytes` have been exceeded, or other network errors. Usually, it doesn't make sense to retry a request that resulted in a `NetworkError`, but for `fetch` `keepalive` it _does_ make sense.

Keepalive can be used by multiple third party tools to report e.g. performance data, each of them might be racing to be the first not to fall into the limit. But once the `fetchGroup` is empty again, why not allow the user to try it again _if_ the request error'd because of exceeding `inflightKeepaliveBytes`?

With the current spec, it is not possible to differentiate between network errors that were triggered because of an actual _network_ issue, or those triggered by a self-issued _client limitation_.

At the moment, in chromium and webkit the `fetch` call rejects with a `TypeError` (message for webkit: "Load failed", for chromium: "Failed to fetch"), which is also rather unspecific and could be any arbitrary error.

I think it makes sense to provide more information here to allow the user to recover. I'd be happy to hear opinions.

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