Currently when trying to edit and entity and give it a sitelink that is already used on another entity the messages in the error are returned such as below.
[name] => wikibase-error-sitelink-already-used
[type] => error
[parameters] => Array(
 => //de.wikipedia.org/wiki/Sacha_Baron_Cohen  => Sacha Baron Cohen  => Q29055 )
This gives the full url, title and entity.
It would make a lot of sense to also have another parameter of the siteid!!!