When a referenced user in field X is deleted, the entity that contains field X can't be saved or deleted

To reproduce:
- create a new node type N
- create a user reference field in N
- create a new user U
- create a new node of type N
- reference U in the user reference field in the node
- now delete U
- now try to either save or delete the new node
- you will get an error 'this user can't be referenced.'

I believe the same problem exists with other reference types, such as node or taxonomy.

To fix:
Delete references when the referenced objects are deleted. David Rothstein's approach seems to be a good one: http://drupal.org/sandbox/drothstein/1775816

I am using this module now but it would be better if it were a part of entityreference.

Regards,
Bas