Word wrap "No backends found" error message

Summary: This makes the complete error message visible if the window is small, for example on Plasma Mobile

Test Plan: Error message is word wrapped.

Reviewers: #discover_software_store, ngraham

Reviewed By: #discover_software_store, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20674