[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