ItemSync: skip handling remote items if local changes failed

Summary:
The infamous "Multiple merge candidates" error would still leave ItemSync
in a forever-stuck state when mRemoteItemsQueue was not empty.

Reproduced by adding a unittest that calls setFullSyncItems (with
a duplicate item), while the existing unittest for a duplicate item
was callling setIncrementalSyncItems().

I barely understand what I'm doing, please review carefully.

Test Plan: Unittest passes.

Reviewers: dvratil, vkrause

Reviewed By: dvratil

Subscribers: asn, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20243