xwayland/glamor: Change errors to verbose messages

On a normal startup sequence, the Xwayland glamor backend would log
an error whenever a required Wayland protocol is missing.

Those are not really errors though, more informational messages along
the glamor backend selection process.

Demote those errors to verbose messages to reduce the verbosity of
Xwayland at startup by default.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>