Bug 37510: Fix Objects.t and remove FIXME

Bug 37510: Fix Objects.t and remove FIXME

This patch makes the tests check for the added exception instead of the
generic DBIx::Class::Exception.

A FIXME was set in place proposing we should throw a Koha::Exception
instead, so removing the FIXME as that's what this patch actually did.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 8da67cec7217c4519fb0a73d33c65e967ecef529)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>