Summary:
If KWin fails to start the Wayland server due to XDG_RUNTIME_DIR not
being set, kwin_wayland should terminate with an error condition but
not crash.

This change makes sure that KWin detects that the Wayland server does
not work and terminates the startup early and ensures that it doesn't
crash while going down.

An error message is shown that we could not create the Wayland server.

Test Plan:
Test case added which verifies that WaylandServer fails to
init. Manual testing that kwin_wayland exits with error 1.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2078