bsdinstall: improve checksum mismatch error for snapshots

The usual case of a mismatched checksum for installer snapshots
(e.g., -CURRENT, -ALPHA*) is that a newer snapshot has been built and
the old install sets have been replaced. Provide a specific error
message for checksum mismatches there that suggests looking for a newer
snapshot.

Submitted by: Guangyuan Yang <yzgyyang@outlook.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11641