periodic: fix exit status for nonexistent arguments

When called with an absolute pathname, periodic should attempt to execute
every script in that directory. If the directory does not exist, it should
print an error and exit 1. Due to a copy/paste mistake in r231568, it exits
0 in that case.

Reported by:	devel/hs-ShellCheck
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D13070

usr.sbin/periodic/periodic.sh

@@ -103,7 +103,7 @@ case $arg in
         dirlist="$arg"
     else
         echo "$0: $arg not found" >&2
-        continue
+        exit 1
     fi
     ;;
 *)  dirlist=