Bug 12823: Alert about defining the SRU search field mappings

Bug 12823: Alert about defining the SRU search field mappings

This is a follow-up for report 6536 (SRU search targets).
It will alert a user that saves a SRU server without field mappings.

Test plan:
Add a Z39.50 server. No confirm message.
Add a SRU server without field mappings. Cancel the confirm.
Add one field mapping. No confirm message.

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>