Bug 29797: Fix display of itemnumbers on the background job detail view

Bug 29797: Fix display of itemnumbers on the background job detail view

If some items have not been deleted, the detail view of the background job will display "1 item(s) could not be deleted: ", without the list of itemnumbers.
Also add a space character between itemnumbers.

Test plan:
1. Check an item out
2. Use the batch delete items tool to delete the item that is checked
out and another one.
3. On the confirmation view, use the dev tool of your browser to remove
the "disabled" property of the disabled checkbox and select both
items.
4. Submit
5. Go to the detail view of the background job and confirm that you see
the itemnumber of the item that has failed to be deleted

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>