Improve the error message for MERGE statements with map property parameters

This change has the desired effect. Running the query [here](https://github.com/neo4j/neo4j/issues/975) now gives a helpful error message.

But the implementation seems a bit hacky, depending on the special value "*" for the property key. Is there a better way of doing this? Should the key have its own type to carry this information more securely?

@systay, @boggle, @thobe: thoughts?