freebsd-update: Add some diagnositic information for a failure case

Users report freebsd-update failing with "The update metadata index is
correctly signed, but failed an integrity check." Add a hint at which
of the cases is failing to help track down the issue.

PR: 264205
Reviewed by: dch
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52222