don't mark already non-interceptible builds
don't mark already non-interceptible builds

also simplify build status check; if it's completed and not
aborted/failed/errored, it can only be succeeded, which seems clearer as
that's more strictly the case where we don't want to intercept

concourse/concourse#1218

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse