Print an explicit error if the backup volume is not present in

hw.disknames. This can only happen due to a failure or user error.
In either case, silent failure makes it hard to discover and debug.
Now it will be easy to spot in the daily mail.
ok rob, schwarze

src/etc/daily

@@ -1,5 +1,5 @@
 #
-#	$OpenBSD: daily,v 1.90 2017/07/10 11:18:48 bluhm Exp $
+#	$OpenBSD: daily,v 1.91 2018/02/06 19:57:37 tb Exp $
 #	From: @(#)daily	8.2 (Berkeley) 1/25/94
 #
 # For local additions, create the file /etc/daily.local.
@@ -90,7 +90,10 @@ while [ "X$ROOTBACKUP" = X1 ]; do
 	fi
 	rootbak=${rootbak#/dev/}
 	bakdisk=${rootbak%%?(.)[a-p]}
-	sysctl -n hw.disknames | grep -Fqw $bakdisk || break
+	if ! sysctl -n hw.disknames | grep -Fqw $bakdisk; then
+		echo "Backup disk '$bakdisk' not present in hw.disknames."
+		break
+	fi
 	bakpart=${rootbak##$bakdisk?(.)}
 	OLDIFS=$IFS
 	IFS=,