freebsd-update: Clarify unsupported upgrade message

PR:		204115
Submitted by:	Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by:	delphij
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18916

usr.sbin/freebsd-update/freebsd-update.sh

@@ -1022,7 +1022,16 @@ fetch_pick_server () {
 
 # Have we run out of mirrors?
 	if [ `wc -l < serverlist` -eq 0 ]; then
-		echo "No mirrors remaining, giving up."
+		cat <<- EOF
+			No mirrors remaining, giving up.
+
+			This may be because upgrading from this platform (${ARCH})
+			or release (${RELNUM}) is unsupported by `basename $0`. Only
+			platforms with Tier 1 support can be upgraded by `basename $0`.
+			See https://www.freebsd.org/platforms/index.html for more info.
+
+			If unsupported, FreeBSD must be upgraded by source.
+		EOF
 		return 1
 	fi