Improve error message when referencing undefined metadata elements

[akhtars]

If I attempt to set the value of a metadata element that hasn't been defined in the terminology, such as fancybook.name.give_name = "Zoia" (note typo) instead of fancybook.name.given_name = "Zoia", (example from this lesson), I get this error message:

NoMethodError: undefined method `sanitize_new_values' for nil:NilClass
    from /home/shaun/.rvm/gems/ruby-2.0.0-p576/gems/om-3.1.0/lib/om/xml/dynamic_node.rb:76:in `val='
    from /home/shaun/.rvm/gems/ruby-2.0.0-p576/gems/om-3.1.0/lib/om/xml/dynamic_node.rb:64:in `new_update_node'
    from /home/shaun/.rvm/gems/ruby-2.0.0-p576/gems/om-3.1.0/lib/om/xml/dynamic_node.rb:43:in `method_missing'

It would be helpful to provide a message indicating that the error is due to not finding the referenced element in the terminology.