V1.2.0 | DS18B20 Temp Error Codes

https://community.openenergymonitor.org/t/emonpi-temperature-measurement/6792/17

Implement standard temp error codes:

#define UNUSED_TEMPERATURE 300
// this value (300C) is sent if no sensor has ever been detected
#define OUTOFRANGE_TEMPERATURE 302
// this value (302C) is sent if the sensor reports < -55C or > +125C
#define BAD_TEMPERATURE 304
// this value (304C) is sent if no sensor is present or the checksum is bad (corrupted data)
// NOTE: The sensor might report 85C if the temperature is retrieved but the sensor has not been commanded