[baloosearch] Bail out early if specified folder is not valid

Test Plan:
$> baloosearch -d /foo "test"
/foo is not a valid directory

Reviewers: Baloo, ngraham, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28502