Proper exception message when using a wrong attribute mapping for referenced IdentityType

When using two identity stores, and only one of them supporting attributes (ad-hoc), the following exception is raised when the entity being used to store attributes has a wrong mapping:

throw new IdentityManagementException("Could not find mapper for attributes for type [" + attributedType + "].");

This happens because the entity does not provide a String-based field to store the referenced IdentityType. In this case we should use a more specific message.