Improve error message when XML element is removed

When an element is removed from the XML configuration schema, configurations using the old XML namespace trigger a warning that the element is deprecated, including the name of the replacement element.

Configurations files with no namespace declarations are assumed to be using the latest schema and don't trigger the warning, instead they trigger a generic `Unexpected element 'X' encountered` exception.

Assuming the latest schema is the right thing to do, but we need to improve the error message for upgraders.