Bug 12922 - Do not DIE the advance_notices.pl -cronjob if no letter of type is found!!

Bug 12922 - Do not DIE the advance_notices.pl -cronjob if no letter of type is found!!

We failed to deliver advance_notices because a template for sms's is undefined, because we don't support
sending sms' as advance_notice.
This crashed the cronjob because digests are set to die instead of the warn used in non-digest.
And we get angry customers asking for compensation!

This patch replaces the die with warn.

TEST PREPARATION:

0. Edit the ODUEDGST letter, find an undefined letter for any trasport type.

TEST PLAN:

1. Find a borrower and from the messaging preferences set the "Advance notice" transport type to
   the undefined digest. Set the "Days in Advance" to 1.
2. Check-out something for that borrower and set the due date for tomorrow.
3. Run "misc/cronjobs/advance_notices.pl -c -n -v" from the terminal.
4. BEFORE THIS PATCH: You get an error
   "No circulation PREDUEDGST letter transported by sms at /home/koha/kohaclone/C4/Letters.pm line 609."
   and the script dies.
4. AFTER THIS PATCH: You get an error
   "No circulation PREDUEDGST letter transported by sms at /home/koha/kohaclone/C4/Letters.pm line 609."
   but the script keep on going!

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>