freebsd-update: Clarify unsupported release upgrade error message

Notify users that upgrading from -CURRENT or -STABLE is unsupported by
freebsd-update.

Also ensure --currently-running provides a correctly formatted release
(as done by -r).

PR: 234771
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reported by: yuri
Reviewed by: bcran
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18803