Exit out if nvidia-smi is not present in PATH for production mode.

[deepujain]

1. nvidiagpubeat can be run in production mode.
2. In case nvidia-smi is not in PATH, then throw appropriate error message and exit

Possible solution

1. gpu.go command() must return err object for above scenario.
   2.metrics.go must check on error and work accordingly.

if err != nil return err ```

Ex

2019-09-04T12:32:57.008-0700 INFO instance/beat.go:400 nvidiagpubeat start running.
2019-09-04T12:32:57.008-0700 INFO beater/nvidiagpubeat.go:57 nvidiagpubeat is running for ** production ** environment. ! Hit CTRL-C to stop it.
2019-09-04T12:32:58.038-0700 ERROR nvidia/gpu.go:52 E! nvidia-smi is not in PATH. Exit !!!