Accessing to a texture which doesn't exist stop loading

[Vinc3r]

Example: if a bumpTexture is not assigned on the material, this patch will crash the loading (and doesn't give useful infos in the browser console)

[
    {
        "*default*:material":
        {
            "ambientColor": "red",
            "bumpTexture":
            {
                "level": 1
            }
        }
    }
]