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().

Test Plan: Unittest passes.

Reviewers: dvratil, vkrause

Reviewed By: dvratil

Subscribers: asn, kde-pim

Tags: KDE PIM

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

(cherry picked from commit 257598195d09b1022ebf08cd58245cd3493f5a2e)