Re: [PATCH v2] Adding stressor validation for stress-ng

Hi Sana,

I have one suggestion about error handling:

**Issue: Silent failure when stress-ng is not installed**

In `get_valid_stressors()`, when stress-ng is not found, the function returns an empty list:

```python
except (subprocess.CalledProcessError, FileNotFoundError):
    return []
```

Then in `validate_stressor()`, this empty list causes validation to silently pass:

```python
if not valid:
    return  # Allows any stressor name through
```

**Suggested fix:**

```python
def get_valid_stressors():
    """Query stress-ng for list of valid stressor names."""
    try:
        result = subprocess.run(['stress-ng', '--stressors'],
                                capture_output=True, text=True, check=True)
        return result.stdout.strip().split()
    except FileNotFoundError:
        print("stress-ng is not installed. Please install the stress-ng package.")
        sys.exit(1)
    except subprocess.CalledProcessError as e:
        print(f"Failed to query stress-ng stressors: {e}")
        sys.exit(1)
```

This follows the pattern used in rteval/cpupower.py and gives users a clear error message during `_WorkloadPrepare()` instead of a cryptic failure later.

Thanks
John Kacur