Check if grive didn't finished and print error message to sdout. Crontab will then mail you with postifx (https://easyengine.io/tutorials/linux/ubuntu-postfix-gmail-smtp/).

grive.sh

#!/bin/bash
grive -p /home/simon/grive >> /tmp/grive.log 2>&1
if tail -1 /tmp/grive.log | grep -qv 'Finished!'; then
  echo "There was en error executing grive. check /tmp/grive.log!"
fi