Map NEL phases to fetch

[miketaylr]

NEL phase	Fetch concept / step / algorithm or w/e
DNS resolution	https://fetch.spec.whatwg.org/#resolve-an-origin
Secure connection establishment	https://fetch.spec.whatwg.org/#concept-connection-obtain
Transmission of request and response	https://fetch.spec.whatwg.org/#concept-main-fetch

[miketaylr]

We can be descriptive like we currently are, or we can set a "phase" on something (request? network request?):

DNS resolution, something like, "If the result of [=resolving an origin=] is failure, set phase to "DNS resolution".

Secure connection establishment: "If the result of [=obtaining a connection=] is failure, set phase to "Secure connection establishment".

Otherwise, "Transmission of request and response" (because you're inside of fetch at that point).

Reading more - it seems we already set the phase on "network error". So maybe this is duplicative, or maybe just being descriptive is fine.