Add method name missing to raised error message

Tells you what method you've not implemented, and in which formatter.
You could use `assert_raise_with_message` to avoid having two assertions: http://www.ruby-doc.org/stdlib-2.1.2/libdoc/test/unit/rdoc/Test/Unit/Assertions.html#method-i-assert_raise_with_message. Not sure you have it in your version.