QWaylandDisplay: Correctly intercept all errors when dispatching.

A connection reset isn't the only form of error we may run into, so make sure we
check for other exceptional circumstances through wl_display_get_error.

This fixes my case of e.g. wl_drm throwing an error but QtWayland never
quitting the client.

Change-Id: I8c76dd7913640e58d03bd2fe52eb054a4daa0235
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
QWaylandDisplay: Centralize error handling in one place.

Change-Id: Ifba23e349a4006cea501480a7230408f0fafd1b7
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>