[AppStream Runner] Pass on error message from the library

Summary:
AppStreamQt since 0.10.6 allows to pass on error messages in case some
errors occured when loading the AppStream data.

CCBUG: 374279

Reviewers: #plasma, apol, ngraham

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14804