Sometimes it can be that `today_eq` is reset when the day switches but
it has not been added to `total_eq` yet. The `total_eq` should always be
non-decreasing, so return the last known value until this is corrected
(this most suredly happens during the night).
Also, allow for `login` and `update` to mutate the state of the service
to be able to update things like the last known total produced energy
value.
* Also deserialize the status (error) code and message
* Handle `data` fields having the value `""` in API responses if there
is an error
* Add missing documentation for API struct fields
