Interactive comics don't work

Interactive comics (like http://xkcd.com/1663) don't have an image, so they fail to load.
If a comic is interactive (this can be checked by looking at the "img" property: if it equals "http://imgs.xkcd.com/comics/", it's interactive), the app shouldn't throw an error, but show a useful message to the user.

I see two different approaches: either we just show a simple message, linking to the webpage, or we show the page in the app.
The latter case would be the better option (by far), but it's also hard to realise: we would need to include WebKit in the app (making sure cookies etc. are stored between sessions), and ideally only show the relevant area (only the comic itself), if at all possible with WebKit.

Setting as Wishlist, because as soon as https://bugs.launchpad.net/xkcd-elementary/+bug/1590741 is fixed, errors will be reported to the user, hopefully informing the user of any interactive comics.