Broken UUID broke whole section on admin side

[zerc]

If we get broken UUID which don't pass UUID_WITH_EFFECTS_REGEX check - then we got 500 error in production because:

class FileField(six.with_metaclass(models.SubfieldBase, models.Field)):
    """Django model field that stores uploaded file as Uploadcare CDN url.
    """
    def to_python(self, value):
        if value is None or value == '':
            return value

        if isinstance(value, File):
            return value

        if not isinstance(value, six.string_types):
            raise ValidationError(
                'Invalid value for a field: string was expected'
            )

        try:
            return File(value)
        except InvalidRequestError as exc:
            raise ValidationError(
                'Invalid value for a field: {exc}'.format(exc=exc)
            )

Raised ValidationError from to_python method bring us 500 error.

Simple case (used test_project):

from gallery.models import *

photo = Photo.objects.all()[0]
photo.arbitrary_file.uuid += 'ad'  # Will be nice if we raise exception in this point
photo.save()

And now if we try to open just section /admin/gallery/photo/- we catch error page.

I think we need change File.uuid to descriptor. Then we get full control over changing it value:

photo.arbitrary_file.uuid += 'ad'  # exception. We can't change it on invalid value