Silence updateGravity job unless errors occur

No need for append/logrotate as we are only interested in the latest output
Signed-off-by: jeremysherriff <jeremysherriff@gmail.com>

advanced/Templates/pihole.cron

@@ -16,7 +16,9 @@
 
 # Pi-hole: Update the ad sources once a week on Sunday at a random time in the
 #          early morning. Download any updates from the adlists
-59 1    * * 7   root    PATH="$PATH:/usr/local/bin/" pihole updateGravity
+#          Squash output to log, then splat the log to stdout on error to allow for
+#          standard crontab job error handling.
+59 1    * * 7   root    PATH="$PATH:/usr/local/bin/" pihole updateGravity >/var/log/pihole_updateGravity.log || cat /var/log/pihole_updateGravity.log
 
 # Pi-hole: Flush the log daily at 00:00
 #          The flush script will use logrotate if available