Silence updateGravity cron job unless errors occur

[jeremysherriff]

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What does this PR aim to accomplish?:
Quieten the updateGravity cron job for successful runs.
Fixes #2424

How does this PR accomplish the above?:
Update the Pi-hole cron file to pipe all stdout to a log file.
Display all logged output to stdout if exit code not zero, via || command linking.
Requires minor tweak of pihole master script to return exit code from gravity.sh to cron parent process.

What documentation changes (if any) are needed to support this PR?:
None. Cron file contains comments to explain the more complex command structure.

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[jeremysherriff]

This would have been better handled by adding a quiet flag for the gravity.sh script in the same way that the flush script has, squashing any non-error output. However the way that gravity's blocklist fetch status output is written makes that very difficult (the use of echo -ne to replace pending with success/failure) and it is beyond my capabilities to retrofit such complex message handling 😄
Maybe one day someone could rewrite gravity to factor in a genuine quiet flag.

[mitchellrj]

Thank you very much for this - it's been bothering me for a while so I finally got around to looking into fixing it and found you already had! :)

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/make-output-of-cron-jobs-suppressible-have-no-output/1551/5