Inform the user if the module can not be registered with org.kde.kded5 and exit with error

Summary:
It makes no sense to run this test app without registering with kded5
as this is a required requirement.

Test Plan: compiled and tested without running kded5

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9453