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

(cherry picked from commit af4ba95daf75cf1b1624dd57038cfaa3ed2753e7)
(cherry picked from commit d653eca0ef889a8e3fa1e370d2b3ce7b031441a2)