Bug 32379: Add check on existing item

Bug 32379: Add check on existing item

Simplest fix; bail out with output_error.

Test plan:
Try /cgi-bin/koha/cataloguing/additem.pl?biblionumber=1&op=saveitem&itemnumber=999999
Note: Replace 1 by existing biblionumber, and iitem 999999 should not exist.
You should get the 404 screen now.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>