Port hotplug from KRun::runCommand to CommandLauncherJob.

Summary:
Is there a better way to report error from this code than the dialog box
that KRun used to pop up (and KDialogJobUiDelegate does too)?

Test Plan: Untested.

Reviewers: davidedmundson, sitter, broulik

Reviewed By: broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28988