Bug 28582: Fix hashref in a warning message

Bug 28582: Fix hashref in a warning message

Can't enqueue letter HASH(0x55edf1806850)

Test plan:
If you cancel an article request when there is no email address
(no branch email, no KohaAdminEmailAddress), Koha warns.
Verify that the warning now contains the letter code.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>