util: set errno when hitting WL_MAP_MAX_OBJECTS

Callers may check errno when wl_map_insert_* functions return an
error (since [1]). Make sure it's always set to a meaningful value
when returning an error, otherwise callers might end up checking an
errno coming from a completely different function.

[1]: https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/205



Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: b19488c7 ("util: Limit size of wl_map")