Prepare fix - Error while updating search collection

Summary:
Current theorie, if the error:
Cannot add or update a child row: a foreign key constraint fails
(akonadi.collectionpimitemrelation,

CONSTRAINT `collectionpimitemrelation_ibfk_2`
FOREIGN KEY (`PimItem_id`)
REFERENCES `pimitemtable` (`id`)
ON DELETE CASCADE
ON UPDATE CASCADE)"

Collection::addPimItem is performed for items in PimItem, that do not exist anymore.
This patch added a debug message, what items are not vaild anymore in PimItem.

Test Plan:
mollekopf tests this patch on this installation and give results back,
if we hit the correct part to fix it.

Reviewers: mollekopf

Maniphest Tasks: T1226

Differential Revision: https://git.kolab.org/D150