Expose details for appcache error events.

Make AppCache "error" events implement a ApplicationCacheErrorEvent
interface, with |reason|, |url|, |status| and |message| properties
to enable recording diagnostics from the field.

BUG=342555

Review URL: https://codereview.chromium.org/164873003

git-svn-id: svn://svn.chromium.org/blink/trunk@170389 bbb929c8-8fbe-4397-9dbb-9b2b20218538