Made XML parsing routines raise an exception if the document is valid XML, but not valid as an OSM document. This is now the same behaviour as when the document isn't valid XML.

app/models/relation.rb

@@ -33,6 +33,7 @@ class Relation < ActiveRecord::Base
       doc.find('//osm/relation').each do |pt|
         return Relation.from_xml_node(pt, create)
       end
+      raise OSM::APIBadXMLError.new("node", xml, "XML doesn't contain an osm/relation element.")
     rescue LibXML::XML::Error, ArgumentError => ex
       raise OSM::APIBadXMLError.new("relation", xml, ex.message)
     end