add_xmlid should check that xmlid does not exist on another model

[guewen]

The add_xmlid method currently skips the creation of the xmlid if it already exists on a different record. It should fail instead. https://github.com/camptocamp/anthem/blob/3dc8e3965e31b0195221e766367ef225fa275735/anthem/lyrics/records.py#L9-L28