Use qWarning and _exit() instead of qFatal for wayland error

This type of error is likely to happen upon system logout. qFatal would
trigger sigabrt and leave unnecessary coredump on the system. Using
qWarning here would make it consistent with xcb's io error.

Pick-to: 5.15 6.0 6.1
Change-Id: I571ba007bf2453486b81837cccdbefa5f181b63d
Reviewed-by: David Edmundson <davidedmundson@kde.org>