Don't warn for invalid plugin metata
Summary:
See explanation in https://bugs.kde.org/show_bug.cgi?id=412464
BUG: 412464
FIXED-IN: 5.67
Test Plan: restart plasmashell, see way less useless warning spam
Reviewers: Plasma, mart, apol
Reviewed By: apol
Subscribers: apol, alex, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D27080