delegate to_xml (with options) to the node

Instead of defining `to_xml` as `node.to_s` call `node.to_xml`, which also accepts an options hash to control the output.  With this change the `to_xml` method for errors behaves just like any other stanza's `to_xml` method.
The difference between `to_xml` with the formatting option and `to_s` is whitespace.  The string produced with `to_xml(:save_with => AS_XML)` strips off any whitespace between tags whereas `to_s` keeps the whitespace between tags intact.