Housekeeper: Validate configuration on start (follow up)

What does this MR do and why?

Follow up from !155328 (comment 1936034843) to improve the code.

I also made the change to check all config keys in one go and print out the error, so that devs will not have to waste time setting up one key, running the keep again, and only then getting to know that another key is missing.

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Before	After

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