Don't get stuck in DavGroupwareResource::onItemFetched, on parse error

Summary:
Sometimes the resource fetches an object (URL to a .ics file) that doesn't exist anymore
(deleted on server, for instance). The error printed a warning but didn't cancel the current task
so the resource would get stuck in that state forever.

Test Plan: Syncing my kolab account continues after printing this error

Reviewers: winterz, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D27858