feat(config): toml deserialization errors

[snormore]

This PR updates the TOML config provider to error when deserialization fails, where previously it would just return the default of the config.

[ozwaldorf]

Does struct level #[serde(default)] as implemented on most config types (to reuse old fields while still defaulting new ones) silently defeat this?

[ozwaldorf]

We also somewhat rely on the defaulting behavior in lightning-node print-config (ie, to update a config, just lightning-node print-config > ~/.lightning/config.toml)

But maybe that command is not relevant anymore if we do want to require config to always be correct (ie, print config = cat)

[snormore]

It returns the default config if the key isn't found, so print-config still works, and there are a bunch of tests that depend on it too (I tried erroring to see what would break). This only changes the behavior when deserialization of something existing fails.

[ozwaldorf]

Sounds good 👍