better error reporting for co2

main/co2_sensor.c

@@ -36,6 +36,10 @@ static int mb_write_and_read(uint8_t *buffer, int num, size_t resplen) {
     num = uart_read_bytes(CO2_UART_NUM, buffer, resplen, pdMS_TO_TICKS(500));
     ESP_LOGD(TAG, "Received %d bytes", num);
     ESP_LOG_BUFFER_HEXDUMP(TAG, buffer, num, ESP_LOG_DEBUG);
+
+    if (num == 0) {
+        ESP_LOGE(TAG, "No modbus response!");
+    }
     return num;
 }