Bug 7468: Add warning when range has non-existent barcodes

Bug 7468: Add warning when range has non-existent barcodes

+ Added a warning when the user selects a range with some non-existent barcodes, and a layout without barcodes.
This allows the user to print barcodes of items not in the database if he so desires, but will warn him if he tries to print only biblios (which results in missing entries or a blank page).

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>