Resolve issues when nouveau is used and improve error handling

[alberts-s]

This PR:

• Ensures that the inability to read some values does not fail the whole script. If there is an issue, a message will be printed to stderr.
• Returns 0 whenever a value cannot be accessed (e.g in case when nouveau is used)
• Improves error handling in read_parse to be more precise
• Refactors the code to use a main function

An example output of the script in case noveau is in use, but GPU is disabled:

{
...
  "hwmon4-nouveau": {
    "in0": {
      "input": 0,
      "label": "GPU core",
      "max": 1200,
      "min": 600,
      "sensor_type": "in"
    },
    "temp1": {
      "auto_point1_pwm": 100,
      "auto_point1_temp": 90000,
      "auto_point1_temp_hyst": 3000,
      "crit": 105000,
      "crit_hyst": 5000,
      "emergency": 135000,
      "emergency_hyst": 5000,
      "input": 0,
      "max": 95000,
      "max_hyst": 3000,
      "sensor_type": "temp"
    }
  }
}