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 012325e3)