Port away from a deprecated signal

Summary:
QProcess::error() has been deprecated since Qt 5.6. It is highly advised
to use QProcess::errorOccurred() instead.

Test Plan: Compiles.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26922