Throw from xml::node::get_type() if the node type is unknown
Throw from xml::node::get_type() if the node type is unknown

Don't just return type_element for any unknown/unexpected types, but
throw an exception instead. In practice, this shouldn't catch anything
because all the values other than those already handled should be
impossible anyhow, but this makes the code more robust in case more node
types are added in the future and allows to globally enable the useful
-Wswitch-enum gcc warning when building the library.

vslavik/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp