Only test NextNodeUndefined exception message once
Only test NextNodeUndefined exception message once

There were two tests asserting against the message of this exception.

While it's worthwhile testing that both scenarios raise the exception,
it's not worth asserting against the message in both tests.

alphagov/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers