Alexa SmartHome: Dry mode

[Thunderbird2086]

Operating environment (Hass.io/Docker/Windows/etc.):
Hassbian

Component/platform:
alexa and climate

Description of problem:

User: Alexa, set aircon to dry
Alexa: Hm, Aircon is not responding.

Additional information:
It seems there are two different issues here.

The first is definition of component.alexa.const.API_THERMOSTAT_MODES. Per ThermostatMode, Alexa supports only AUTO, COOL, HEAT, ECO and OFF. However, Alexa actually sent CUSTOM for the dry mode as shown below.

'payload': {
   'thermostatMode': {
       'value': 'CUSTOM',
       'customName': 'DEHUMIDIFY'
   }
}

The second is Alexa response. Its error type is not listed on Alexa.ErrorResponse Interface. I think this is why Alexa said 'aircon is not responding'

'payload': {
  'type': 'UNSUPPORTED_THERMOSTAT_MODE',
  'message': 'The requested thermostat mode CUSTOM is not supported'
}

Please take a look and fix it if needed.