spec: add slack alerting for failures

closes: CCI-40

Change-Id: I78a022754cb6cff0c40e9d7a2413ff204fd20bf6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/218003
Tested-by: Jenkins
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Derek Bender <djbender@instructure.com>
QA-Review: James Butters <jbutters@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>

Jenkinsfile

@@ -356,6 +356,15 @@ pipeline {
   }
 
   post {
+    failure {
+      script {
+        if ( env.GERRIT_EVENT_TYPE == 'change-merged' ) {
+          slackSend (channel: '#canvas_builds',
+            color: '#da0005',
+            message: "${env.JOB_NAME} failed on merge (<${env.BUILD_URL}|${env.BUILD_NUMBER}>)")
+        }
+      }
+    }
     cleanup {
         sh 'build/new-jenkins/docker-cleanup.sh'
     }