Make sure qmlplugindump works on my system
Summary:
Using setWorkingDirectory made QProcess fail upon start, this way it works
nicely.
Also adds some useful debug info and causistic in case it fails again.
Reviewers: KDevelop, mwolff
Reviewed By: KDevelop, mwolff
Subscribers: kdevelop-devel
Differential Revision: https://phabricator.kde.org/D10782
(cherry picked from commit 012325e35a43323aee1c518f60273ecc34f7a4b8)