Retrieves the latest exchange rates from Open Exchange Rates site.

• app_id is a valid Open Exchange Rates app_id. Defaults to the configured app_id in config.exs

Returns:

• {:ok, rates} if the rates can be retrieved

• {:error, reason} if rates cannot be retrieved

Typically this function is called by the exchange rates retrieval service althouhg it can be called outside that context as required.