Use JsonSchema to parse config.json to give better error messages

[alexbowe]

This should probably apply whenever the config changes (before allowing the change)

[abdnh]

Anki provides that via a config.schema.json file. The config is validated when the user modifies the config from the config screen at least: https://github.com/ankitects/anki/blob/82caffecbcdabc52f07fe98a6d429ede96bf8eb1/qt/aqt/addons.py#L1604