Bug 6963: Corrects a problem when a new order is created with a duplicate barcode.

Bug 6963: Corrects a problem when a new order is created with a duplicate barcode.

If the AcqCreateItem preference is set to "ordering" and the barcode for
the new item is already in use, no error is returned, but an invalid
itemnumber is saved in the aqorders_items table and the item is never
created.

This patch adds a duplicate barcode verification in neworderempty.pl
_koha_add_item is also modified so it won't return an invalid ID when
an item can't be added.

http://bugs.koha-community.org/show_bug.cgi?id=6963
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Test plan on second patch.