// Get a texture with an invalid url
var url = "invalid_url";
function Start () {
// Start a download of the given URL
var www : WWW = new WWW (url);

// Wait for download to complete
yield www;

// Print the error to the console
if (www.error != null)
Debug.Log(www.error);

// assign texture
renderer.material.mainTexture = www.texture;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public string url = "invalid_url";
IEnumerator Start() {
WWW www = new WWW(url);
yield return www;
if (www.error != null)
Debug.Log(www.error);

renderer.material.mainTexture = www.texture;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public url as string = 'invalid_url'

def Start() as IEnumerator:
www as WWW = WWW(url)
yield www
if www.error != null:
Debug.Log(www.error)
renderer.material.mainTexture = www.texture