Exit with failure if an artifact is missing

- Why don't we return an error here? :)

Change-Id: I2ee63371a6a3425206915ee6d6551764bf1b77fd
diff --git a/kokoro/continuous.sh b/kokoro/continuous.sh
index 3f9d6a9..2fadd09 100644
--- a/kokoro/continuous.sh
+++ b/kokoro/continuous.sh
@@ -49,4 +49,6 @@
   cp ${PRODUCT_OUT}/rootfs.img ${KOKORO_ARTIFACTS_DIR}
   cp ${PRODUCT_OUT}/sdcard.img ${KOKORO_ARTIFACTS_DIR}
   cp ${PRODUCT_OUT}/manifest.xml ${KOKORO_ARTIFACTS_DIR}
+else
+  exit 1
 fi