Handle exception on nvidia driver fail

[wasndas]

On nvidia driver fail the systemd service quits after several retries, catching the exception whould be great.

Traceback (most recent call last):
  File "/usr/bin/superfans-gpu-controller", line 11, in <module>
    load_entry_point('superfans-gpu-controller==0.1.1', 'console_scripts', 'superfans-gpu-controller')()
  File "/usr/lib/python3.6/site-packages/superfans_gpu_controller.py", line 177, in main
    superfans_cgpu_controller(cfg['fan_settings'], cfg.get('use_ipmi_modes') is not None)
  File "/usr/lib/python3.6/site-packages/superfans_gpu_controller.py", line 89, in superfans_cgpu_controller
    enable_persistance_nvidia()
  File "/usr/lib/python3.6/site-packages/superfans_gpu_controller.py", line 47, in enable_persistance_nvidia
    s = subprocess.check_output(cmd + " 2>&1", shell=True)
  File "/usr/lib64/python3.6/subprocess.py", line 356, in check_output
    **kwargs).stdout
  File "/usr/lib64/python3.6/subprocess.py", line 438, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command 'nvidia-smi -pm 1 2>&1' returned non-zero exit status 255.