Enable wrapping of error messages which use KMessageWidget

Summary: Error messages should be shown to users no matter how much long it is.

Reviewers: #vdg, #plasma, #frameworks, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28200