Add xpath to xsd errors

[markusgeert]

Currently when validating an xml file based on a xsd schema, only line and column numbers are returned. I found this to be hard to work with. On my local fork I therefore added some code to also return the xpath of where the error was found (using the node stored in the err object.) Would this be nice to include in the library? If so, I would be willing to make a pr for this.