Ensure StandardDirs::saveDir returns existing dir

Summary:
This is slight adapted version of the code removed in commit
278a85f8. I believe it got lost
unintentionally in that commit as there are lots of places in current
codebase which relies on StandardDirs::saveDir() returning existing
directory (just check comments in those places!), e.g.:

- AkonadiServer::startDatabaseProcess()
- PartStreamer::PartStreamer
- StorageJanitor::verifyExternalParts()
- etc.

Test Plan:
akonadictl fsck on database with unreferenced external files actually moves
files to file_lost+found dir now.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D17879