Refactor error handling so we do chain the error with the `?`
operator rather than wrapping it with own types.
Links to other error chains are created with error chain. All unneeded
error kinds were removed.
No API changes.
Before we extracted the store configuration from the configuration
toml::Value object and passed it to the store.
This is unecessary overhead.
Now we pass the whole configuration object and let the store extract the
required values.
