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