MTP: fix runtime warning when showing a warning to the user.

Summary:
KIO says the following:
listDir() did not call finished() or error()! Please fix the "kio_mtp" KIO slave

and message boxes are annoying anyway, better use the error mechanism
for actual errors.

Test Plan: Not fully tested I admit, but seems obvious :-)

Reviewers: akrutzler, elvisangelaccio, apol

Reviewed By: akrutzler, elvisangelaccio, apol

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D16674