Display a validation error when vocabulary holds duplicate keys

This will avoid users to have that kind of errors at startup:

2011-12-12 18:50:29,443 ERROR [org.nuxeo.runtime.model.impl.RegistrationInfoImpl] Component notification of application started failed.
org.nuxeo.ecm.directory.DirectoryException: org.nuxeo.ecm.core.api.WrappedException: Exception: org.nuxeo.ecm.directory.DirectoryException. message: Table 'studio_vocabulary_t' initialization failed: The statement was aborted because it would have caused a duplicate key value in a unique or primary key constraint or unique index identified by 'STUDIO_VOCABULARY_T_PK' defined on 'studio_vocabulary_t'., values: [0: foo, 1: foo, 2: 0]