More helpful error message

I've reworked the 'reference overloading' to be more helpful. It now reads like so:

Reference overloading: Node Title (nid: 55) would have had 2 references and only 1 reference is permitted in field_scrum_stories_parent on scrum_user_story. Before adding a reference, you would need to edit Talk to other team members on IRC (nid: 55) to remove an existing reference and resave this node to make it correspond. Alternatively, you could allow this field instance to have more references, by changing the cck field settings of the instance.