[whatwg] AppCache error event details

[Joshua Bell]

We'd like to move forward on adding event details to AppCache errors. [1]

I've fleshed out a proposal [2] that details the additions to the HTML
spec. It introduces a new event type (ApplicationCacheErrorEvent) with
reason (an enum), url, status, and message fields.

Feedback would be appreciated, particularly about what level of information
is safe to expose about cross-origin resource fetches which Chrome does
support with AppCache (as Hixie mentioned in [3]). The fact that a fetch
failure occurred with a specific URL does not appear to be a new piece of
information, but presumably further details (e.g. the specific http
response code, more detailed message) should not be exposed?

[1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=22702
[2]
https://docs.google.com/document/d/1nlk7WgRD3d0ZcfK1xrwBFVZ3DI_e44j7QoMd5gAJC4E/edit?usp=sharing
[3]
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/blSfs6IqcvY/jCfGdH3p8eAJ