torture: Simplify exit-code plumbing for kvm-recheck.sh and kvm-find-errors.sh

This commit simplifies exit-code plumbing.  It makes kvm-recheck.sh return
the value 1 for a build error and 2 for a runtime error.  It also makes
kvm-find-errors.sh avoid checking runtime files for --build-only runs.

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