torture: Make kvm.sh return failure upon build failure

The kvm.sh script uses kvm-find-errors.sh to evaluate whether or not
a build failed.  Unfortunately, kvm-find-errors.sh returns success if
there are no failed runs (including when there are no runs at all) even if
there are build failures.  This commit therefore makes kvm-find-errors.sh
return failure in response to build failures.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>