title: Dataflow job - update_if_exists?
body: <p>I use Google dataflow job. I have following command to deploy job</p>
  <pre><code>java -jar build/libs/my-job-1.0-all.jar \
    --project=$PROJECT \
    --region=us-central1 \
    --subscription=$SUBSCRIPTION \
    --jobName=my-job \
    --runner=DataflowRunner \
    --streaming=true \
    --stableUniqueNames=ERROR \
    --workerMachineType=n1-standard-2 \
    --usePublicIps=false \
    --network=default \
    --update
  </code></pre>
  <p>This works fine when I have running job in GCP but it would fail if there is nothing to update. If I remove update flag it works fine as long as there is no running job.</p>
  <p>Is there a way to specify such that if job exists update it, and not just start a new job?</p>