cursor-sprite/xcursor: Fall back to gray square if missing theme

Instead of aborting with an error, display a half transparent gray
square instead of cursors and log a warning in the journal, allowing the
user to fix their system withotu having to rely on switching to a TTY.

It will be immediately obvious the cursor is silly looking, which will
be a better hint than just aborting.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1428
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1563>

(cherry picked from commit 83360a4a)