Port over most of the old XML error handling.

Some messages are a bit different due to ruby-ll's error handling, other than
that it's largely the same stuff as before.

task/parser.rake

@@ -2,6 +2,10 @@ rule '.rb' => '.y' do |task|
   sh "racc -l -o #{task.name} #{task.source}"
 end
 
+rule '.rb' => '.rll' do |task|
+  sh "ruby-ll #{task.source} -o #{task.name}"
+end
+
 desc 'Generates the parser'
 task :parser => [
   'lib/oga/xml/parser.rb',