Fix error messages on nested attachment validation failures

We used to report things like this:

    Supporting page attachments attachment attachment data file can't be
    blank

Now we say:

    The attached file can't be blank

The need for some (but not all) of this YAML will go away once we drop
all these non-polymorphic relationships between documents of different
types, and Attachment.
Merge pull request #680 from alphagov/attachment-validation-messages

Fix error messages on nested attachment validation failures