deliveryLocationsByBarcode better error handling

[nonword]

1. Endpoint should fail in some way if barcodes param empty/missing. Currently returns an item [mis]serialized with 'urn:barcode:undefined'!

2. While we're at it, endpoint should be more helpful when no barcodes match. Currently returns 500. This isn't a server error. It's a client error if it's an error. Maybe something in the 4** range is correct.