This PR adds detection and log warnings for the following config validation errors:
- Unknown config option (e.g misspelling of a valid config option)
- Config option is a wrong type (e.g. string instead of boolean)

It does *not* propagate this warning to the user (i.e. in onboarding or PRs) yet due to the high chance that we'll find a false negative. I will watch logs for a week or so and then once happy with results will activate user-visible warnings via #556.

Closes #548, Closes #555