Rework "offline page" logic.
Pre-akonadi logic was to check for imap resource and check its offline status, when clicking on a mail. New logic is rather: always try to fetch mail (it could be cached, after all), and only on error, check if resource (of any type) is offline, and then show error page. The main bonus of this patch is that it removes the "early return" in case of offline mode, which led to a much slower folder-switching in the messagelist. Concept by Volker, patch review by Dan.