Bug 14632: Fix alert message for single item batch

Bug 14632: Fix alert message for single item batch

Fixes the incorrect msg "Please select at least label to delete."
for attempts to delete from single patron batches. Notifies the
user that doing so will delete the batch. It disallows direct
deletion, instead suggests the users to use the 'Delete batch'
option assuming the users know what they are doing.

Test plan
=========

1/ Load a single patron batch in edit mode and attempt to delete
   the single record. The JS alert message will inform that "Please
   select at least label to delete."
2/ Apply patch and refresh page and try to delete the single record
   again.
3/ This time the alert will inform the user that doing so will delete
   the batch and should that be desired action to choose the 'Delete
   batch' option from the toolbar.

Signed-off-by: Frederic Demians <f.demians@tamil.fr>
  It works as before but with an understantable warning message.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>