Handle missing "missing" image more gracefully

The "missing image" is a static file and really shouldn't require that hack of copying it to `MEDIA_ROOT`. This raises issues with versioning and is an additional step in project setup. 
Currently, setting up an own Oscar project and forgetting to copy the image throws a IOError. I could imagine letting `MissingProductImage` symlink the file if it's not existent, but am open for better suggestions.