Improved error message for Referencing to the file used in the field is not allowed

Issue Summary

If a content type has many filefield CCK fields, using multigroup for example, it is hard to determine which field is causing problems when getting the following errors:

Referencing to the file used in the %field field is not allowed.
OR
The file referenced by the %field field does not exist.

Amending the error message to include the filename would allow the troublesome files to be identified with greater ease.