Add offline error tracking support

[samgiles]

FYI: Raven.JS now has a primitive available for this to be implemented here: getsentry/sentry-javascript#335 (comment)

cc @pornel useful?

[kornelski]

Useful, and necessary for the webapp.

[kornelski]

We're planning to queue requests using this:

http://git.svc.ft.com:8080/projects/FTA/repos/ft-app/browse/lib/javascript/tracking/store.js

[rowanmanning]

I spoke to @rowanbeentje about this, and offline errors are really low down the app team's priority list at the moment. Considering the lack of activity on this, I'm going to close. We can reopen if/when the app team have a more pressing need for this