[whatwg/fetch] Clarify which network errors create a resource timing entry (#1215)

It's not clear from the spec/implementation which errors should or shouldn't create an RT entry.
CORS errors explicitly don't, though in the original Resource Timing spec they should create an RT entry. However, the discrepancy between different error types might create a case where it could be detected whether an error is of CORS nature, which should be avoided. 

See https://github.com/whatwg/fetch/pull/1202#pullrequestreview-634571341

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