bsdinstall: improve checksum mismatch error for snapshots

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