co2_sensor: Report invalid CO2 measurements

ampel-firmware/co2_sensor.cpp

@@ -216,6 +216,7 @@ namespace sensor {
       if (co2 <= 0) {
         // NOTE: Data is available, but it's sometimes erroneous: the sensor outputs
         // zero ppm but non-zero temperature and non-zero humidity.
+        Serial.println(F("Invalid sensor data - CO2 concentration <= 0 ppm"));
         switchState(BOOTUP);
       } else if (co2 < 250) {
         // Sensor should be calibrated.