Fix ImageInput instead of re-binding image formset

@mbertheau raised a good point that rebinding the image formsets as
added in 0b373df hides any errors in the image formset.
https://github.com/tangentlabs/django-oscar/commit/0b373df5ab76d6fda691fafede0b3e126a91b155#commitcomment-5368878

This fix undoes that change and instead alters behaviour of the
ImageInput widget to not display images that were never persisted to
disk.