mutter should avoid g_error / g_assert where possible
As shown in the bug fixed in !713 (merged) and the bug reported in #703 (closed) we occasionally hit various g_error and g_assert calls bringing the users whole session down.
When mutter was still only a X11 compositor/wm which could be restarted this was ok, but now that mutter also is a Wayland compositor we should really avoid g_error / g_assert where possible (using g_Warning instead) and fail gracefully, so that we do not bring the users session down.