Fixes #87 ('Missing metadata' thrown during ISO validation)

The changes in this pull request should result in more sensible errors being returned to the user when the verification of the ISO associated with an image they are trying to add to Hanlon fails. Without the change in this pull request, all the user receives back from Hanlon in these situations is an extremely generic (and hard to understand) 'Missing metadata' error.  With these changes in place, error strings returned from the underlying ISO verification code are included in the error returned to the user (hopefully this makes for a much more understandable error for the user, since they'll at least know where in the ISO validation process the error occurred).