Add error value for job owner dying

Summary:
When kuiserver detects that a job owner has terminated whilst jobs are
still running we need an error value to send to the client views, such
as plasmashell, to report this error.

Test Plan: None, it's a line in an enum

Reviewers: dfaure, broulik

Reviewed By: broulik

Subscribers: broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17173