fix crash when mapping unified mailboxes to collections

Summary:
when the fetched unified mailbox collections and loaded mailboxes differ then
trying to get a loaded mailbox using the collection name may fail.
wrapping the call in a try-catch block fixes the crash

Reviewers: dvratil, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D19323