Error when DS18B20 temperature sensor is disconnected

[glynhudson]

emonHub displays the following error when a DS18B20 temperature sensor is disconnected:

2026-08-10 14:23:51,013 WARNING  DS18B20    Exception caught in DS18B20 thread. Traceback (most recent call last):
  File "/opt/openenergymonitor/emonhub/src/emonhub_interfacer.py", line 31, in wrapper
    return func(*args)
           ^^^^^^^^^^^
  File "/opt/openenergymonitor/emonhub/src/emonhub_interfacer.py", line 105, in run
    rxc = self.read()
          ^^^^^^^^^^^
  File "/opt/openenergymonitor/emonhub/src/interfacers/EmonHubDS18B20Interfacer.py", line 110, in read
    value = self.ds.tempC(sensor)
            ^^^^^^^^^^^^^^^^^^^^^
  File "/opt/openenergymonitor/emonhub/src/interfacers/EmonHubDS18B20Interfacer.py", line 42, in tempC
    if len(lines[0]):
           ~~~~~^^^
IndexError: list index out of range

EmonHub should gracefully trying to read from a temperature sensor when it's been disconnected.