title: Determine Failed Stage in Jenkins Declarative Pipeline
body: <p>How do I report the stage in which a <a href="https://go.cloudbees.com/docs/cloudbees-documentation/use/reference/pipeline/#_post" rel="noreferrer">declarative pipeline</a> failed?  In the fail block, I want to get failedStage.name and report it (eventually to slack).</p>
  
  <pre><code>pipeline {
      agent { label 'master'}
      stages {
          stage('Ok') {
              steps {
                  echo 'do thing'
              }
          }
          stage('NotOK') {
              steps {
                  sh 'make fail'
              }
          }
      }
      post {
          always {
              echo 'ok'
          }
          failure {
              echo 'Failed during Which Stage?'
          }
      }
  }
  </code></pre>