Bug 9565: (follow-up) Deleting a biblio should alert/fail if there are existent subsc...

Bug 9565: (follow-up) Deleting a biblio should alert/fail if there are existent subscriptions

Clicking on "Delete record" while there are existent subscriptions no
longer delete the record and the subscription. Instead an alert box
inform the user that he must delete all subscription before deleting the
record.

To test:
1) Add a subscription to a biblio, or check a biblio which has one
   attached already.
2) Remove all items if there are any.
3) Apply patch.
4) Confirm that the 'Delete record' item is disabled in the list.
5) Attempt to 'Delete record', you should recieve
   an error message stating to delete all subscriptions before
   deleting record.
6) Sign off.

Sponsored by: Lunds Universitetsbibliotek

Signed-off-by: Samu Heiskanen <samu.heiskanen@hypernova.fi>
Signed-off-by: Frank Hansen <frank.hansen@ub.lu.se>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>