added int return value

[lefty01]

added int return value to verify and begin functions to check if sensor connection failed. I found it useful to have some indication whether a sensor might be attached or not . with this change one could use code like this:

  co2Serial.begin(9600);
  if (0 == co2MHZ19.begin(co2Serial)) {
    co2MHZ19OK = true;
    co2MHZ19.autoCalibration();
  }

not sure if you want to integrate this or not?