Give name of file that generated YAML exception message

It's not much good saying "hey, there was a problem" if you don't know where
the problem *is*.  Hunting through several hundred YAML files is no fun.

lib/jekyll/convertible.rb

@@ -32,7 +32,7 @@ def read_yaml(base, name)
         begin
           self.data = YAML.load($1)
         rescue => e
-          puts "YAML Exception: #{e.message}"
+          puts "YAML Exception reading #{name}: #{e.message}"
         end
       end