Send notification when screencasting fails

Summary:
A notification is sent when something
goes wrong setting up screencasting.

Test Plan: {F8211696}

Reviewers: #plasma, #vdg, ngraham, apol, jgrulich, broulik

Reviewed By: #plasma, #vdg, ngraham, apol, broulik

Subscribers: broulik, apol, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28517