Only activate service account when file exists

hack/jenkins/e2e-runner.sh

@@ -151,9 +151,11 @@ if running_in_docker; then
     fi
 fi
 
-if [[ -n "${KUBEKINS_SERVICE_ACCOUNT_FILE:-}" ]]; then
+if [[ -f "${KUBEKINS_SERVICE_ACCOUNT_FILE:-}" ]]; then
   echo 'Activating service account...'  # No harm in doing this multiple times.
   gcloud auth activate-service-account --key-file="${KUBEKINS_SERVICE_ACCOUNT_FILE}"
+elif [[ -n "${KUBEKINS_SERVICE_ACCOUNT_FILE:-}" ]]; then
+  echo "ERROR: cannot access service account file at: ${KUBEKINS_SERVICE_ACCOUNT_FILE}"
 fi
 
 # Install gcloud from a custom path if provided. Used to test GKE with gcloud