Restart marge-bot on failure

Inspired by !8625 (comment 444389). In that circumstance, marge-bot crashed due to a bug or bad behavior. It would make sense to restart the service when that happens.

We could set Restart=on-failure in the systemd service description.

Currently there is no Restart configured. It used to be Restart=always, but it was turned off in February 2019. @bgamari or @mpickering do you happen to know why it was turned off?

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