Attach timing info and URL to network errors

Also, report timing info for network errors for the fetch() APIs.
The timing info attached to a network error is always "opaque",
containing only start/end time and the original request URL.

This change currently only applies to the fetch API, and should be
applied to the different callers of FETCCH as part of
[this](whatwg/html#6542) and
[this](whatwg/xhr#319) work.

Closes whatwg#1215