[whatwg] AppCache Error events

[David Barrett-Kahn]

Right now this event contains no structured information, just an error
message.  It'd be helpful to us to know more about what failed, so we can
know what to report to the server and take action on.  It's hard to
distinguish cache update failures due to just being offline from those
which are actually causing trouble.  In the second case it's also hard to
work out which resource is proving unavailable and why.

One way to do this would be to create an AppCacheError subclass, with an
errorCode parameter, and also nullable url and httpResponseCode properties.
 Potential error codes:
* couldn't fetch manifest (includes url and httpResponseCode)
* pre and post update manifest fetches mismatched (includes url)
* fetching a resource failed (includes url and httpResponseCode)

Related bug:
https://code.google.com/p/chromium/issues/detail?id=161753

Thoughts?

-Dave