This allows us to use .context instead of .map_err when calling
`latest_rate()`. For the static rate module, we simply fill in
`Infallible` which is actually better suited because it describes
that we are never using this error.
Note that because we are using `watch` channel, only a reference to the
channel value can be returned.
Hence, using custom Error that can be cloned to be able to
pass `Result` through the channel.
