Bug 35969: Add error code in opac-sendbasket.pl

Bug 35969: Add error code in opac-sendbasket.pl

No need to carp about empty basket or no reply address.
Just add a simple explanation on the form.

Note that it makes no sense to show the form if the patron
has no email address. We need it as reply-to.

Test plan:
Try opac-sendbasket.pl?bib_list=null
Remove email address from your account, try again.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>