try { // massive try{} catch{} around the entire build for failure notifications
node('master') {
    stage('Invoke Flake8') {
    // change this to false to fail the job <<<<<
    sh 'true'  
    }
}
} catch (e) {
    currentBuild.result = 'FAILURE'
    throw e
} finally {
        def currentResult = currentBuild.result ?: 'SUCCESS'
        def previousResult = currentBuild.previousBuild?.result
        def SUBJECT = ''
        def BODY = "${env.BUILD_URL}"
        if (previousResult == 'FAILURE' && currentResult == 'SUCCESS') {
            SUBJECT = "Jenkins job ${env.JOB_NAME} #${env.BUILD_NUMBER} fixed."
        }
        else if (previousResult == 'SUCCESS' && currentResult == 'FAILURE' ) {
            SUBJECT = "Jenkins job ${env.JOB_NAME} #${env.BUILD_NUMBER} failed."
        }
        if (SUBJECT != '') {
            emailext to: ownership.job.primaryOwnerEmail,
                     subject: SUBJECT,
                     body: BODY
        }
}