Bug 34598: Improve handling of not found ILL request

Bug 34598: Improve handling of not found ILL request

1) Enable ILLmodule and install FreeForm, run:
   bash <(curl -s https://raw.githubusercontent.com/ammopt/koha-ill-dev/master/start-ill-dev.sh)
2) Visit a request that doesn't exist (assuming empty illrequests table):
   INTRA/cgi-bin/koha/ill/ill-requests.pl?method=illview&illrequest_id=1
3) Verify an error 500 is returned with message "Can't call method
   "backend_illview""
4) Apply patch
5) Repeat steps 1-3. Verify that a "ILL request not found." message is
   now displayed

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>