Bug 18135: Show alert if no authorities selected in batch record deletion

Bug 18135: Show alert if no authorities selected in batch record deletion

To test:
1) Go to Tools -> Batch record deletion
2) Select Authorities as record type, put in an auth id, click Continue
3) Unselect the authorities and click Delete selected records
4) Confirm an alert shows up
5) Clear the alert and confirm deleting the authority still works as
normal

Sponsored-by: Catalyst IT
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>