Bug 21141: Handle non-existent itemnumbers in batch item modification/deletion

Bug 21141: Handle non-existent itemnumbers in batch item modification/deletion

The itemnumber list passed from a file in the batch item
modification/deletion tools is not tested.
Like barcodes we should make sure they refer to a valid items in the DB.

Test plan:
- Create a file with barcodes, itemnumbers and random lines
- Use the batch item modification and deletion tools with this file
You should see a table with a list of invalid barcodes and itemnumbers.

Without this patch you got the following error:
Can't call method "title" on an undefined value at
/home/vagrant/kohaclone/tools/batchMod.pl line 580

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>