rcutorture: Make kvm-find-errors.sh abort on bad directory

Currently, kvm-find-errors.sh gives a usage prompt when given a bad
directory, but then soldiers on, giving a series of confusing error
messages.  This commit therefore prints an error message and exits when
given a bad directory, hopefully reducing confusion.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/bin/kvm-find-errors.sh

@@ -20,7 +20,9 @@
 rundir="${1}"
 if test -z "$rundir" -o ! -d "$rundir"
 then
+	echo Directory "$rundir" not found.
 	echo Usage: $0 directory
+	exit 1
 fi
 editor=${EDITOR-vi}