[balooctl] Add command to show files failed to index

Summary:
Baloo was missing any means to retrieve the list of files which failed
to index.

CCBUG: 406116

Test Plan: $> balooctl failed

Reviewers: Baloo, Frameworks, ngraham, astippich

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20918