Make config optional

[KCaverly]

Handle config errors gracefully, and accept incomplete configs. Provide config healthcheck utilities.

[KCaverly]

This is a good example for what may be helpful:

• https://github.com/mehcode/config-rs/blob/master/examples/hierarchical-env/settings.rs
• https://www.reddit.com/r/rust/comments/qupz5k/how_do_you_merge_config_in_rust_with_a_default/