Introduce proper error type

[gonzolino]

All errors currently returned by gotado are just plain errors. We need a proper error type for gotado that contains the error message, but also the http status code of the tado API. Errors should also properly wrap the underlying error.

[gonzolino]

See https://blog.golang.org/go1.13-errors#TOC_5.

[gonzolino]

Remember that tado° actually returns a list of errors, see e.g.

{
  "errors": [
    {
      "code": "zone.schedule.awayConfig.missingSetting",
      "title": "User setting must be provided when auto adjust is false for heating zone."
    },
    {
      "code": "nullable",
      "title": "Property autoAdjust is required"
    }
  ]
}