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>