add MHZ19X_error_t::none to indicate that measurement values have not yet been retrieved

Author: smdn

examples/demo/I2C/MHZ19XI2C-SLV/src/main.cpp

@@ -17,7 +17,7 @@ uint16_t latestCO2ppm = 0;
 volatile uint8_t getCO2ConcentrationIntervalInSeconds = 5;
 
 // MHZ19XI2C_register_t::error_code
-MHZ19X_error_t getCO2ConcentrationResult = MHZ19X_error_t::success;
+auto getCO2ConcentrationResult = MHZ19X_error_t::none;
 
 // If errors on measurement reading occur consecutively beyond the following count,
 // a software reset using watchdog will be triggered as there could be

src/MHZ19X_error_t.hpp

@@ -5,6 +5,8 @@
 #define MHZ19X_error_t_hpp
 
 enum class MHZ19X_error_t {
+  // Represents a value that is neither success nor failure.
+  none = -1,
   success = 0,
   uart_write_failure,
   uart_read_timeout,