Give more details in the LLCONFLICT error.

Summary:
Admittedly this is not very useful to end users, but it is useful to any
app developer using akonadi.

Example from zanshin:

Cannot deparent task child from its parent: [LLCONFLICT] Resource
akonadi_icaldir_resource_0 tries to modify item 641403
(82d006fc-af0d-40cb-8bbe-a7ff75556358) (in collection 1003) with
revision 5; the item was modified elsewhere and has revision 6, aborting STORE.

Test Plan:
Deparenting a task in zanshin's workday view triggered
this error, due to firing up two ItemUpdateJobs in parallel.

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D9354