Populate Greeter process exit stderr

Summary:
Give more info in debug output when and how the Greeter porcess exited and
what counter-measures are triggered to unregular exits.

Should simplify debugging.

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: davidedmundson, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28081