Bug 29788: Make Koha::Item->safe_to_delete use Koha::Result::Boolean

Bug 29788: Make Koha::Item->safe_to_delete use Koha::Result::Boolean

This patch reuse the (awesome) Koha::Result::Boolean module to retrieve
the return of Koha::Item->safe_to_delete.

Test plan:
Try to delete an item that has previously been checked out and confirm
that you are still blocked.
Try using the cronjobs, the item and biblio detail pages, as well as the
batch delete item tool.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>