Throw an error when invalid device_mode is given (#13739)

* Throw an error when invalid device_mode is given

* Fix lint issue, typo and error msg

* Fix error msg

homeassistant/components/climate/nest.py

@@ -187,6 +187,11 @@ class NestThermostat(ClimateDevice):
             device_mode = operation_mode
         elif operation_mode == STATE_AUTO:
             device_mode = NEST_MODE_HEAT_COOL
+        else:
+            device_mode = STATE_OFF
+            _LOGGER.error(
+                "An error occurred while setting device mode. "
+                "Invalid operation mode: %s", operation_mode)
         self.device.mode = device_mode
 
     @property