Improve the KCM error message

Summary: Improve the KCM error message.

Test Plan: Open a broken KCM. To get one, remove for example the Look and Feel KCM shared library (the file is /usr/lib/x86_64-linux-gnu/qt5/plugins/kcms/kcm_lookandfeel.so).

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: VDG, kde-frameworks-devel, Plasma

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19923