bin/gen_release_notes.py: do not fail on confidential features

bin/gen_release_notes.py: do not fail on confidential features

A commit may refer to an issue marked as confidential. That will look
like a 404 page for outside users. One example of such commit is:

    369c12e5be "anv: clear descriptorsets if AllocateDescriptorSets fails"

Let's handle that case.

Cc: mesa-stable
Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20241>
(cherry picked from commit 334123a908cfac282297ba5240aecac79e9babd2)