[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