Show error messages on the ImageField itself.

This is relevant when you want to require an image for the product by
passing empty_permitted=False to the ProductImageForm.

The result is not particularly pleasing visually but right now the caption field is unintentionally not visible either.
![after](https://f.cloud.github.com/assets/133575/1524476/b90de9ee-4bc6-11e3-87db-e10e67484032.png)