Makes it more consistent with the rest of the `WaylandThread` methods
(so that we can silently check for the window's existence).
Fixes errors when trying to capture the pointer when no surface has been
pointed yet (its logic assumed that this method was silent).
We double-check everywhere anyway but I added it in some places where we
can't either guarantee that it'll check (to avoid segfaults) or that
would be useful to report directly.
874dd28d66