[AppStream Runner] Reduce verbosity of log output

Summary:
Its rather pointless to spew an error message each time a query is done.
Output the warning message just once (on the default logging level), and
output further messages only at debug level.

Reviewers: #plasma, ngraham, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14806