Add option to use the text value for the error msg

Showing the `identifier` may be bad for users understanding where the errors may occur, gives the option to also use `:text`, or custom text for each question:

        question.text = question_text('some question?', identifier)
        question.style = :one
        question.error_subject = :text # or 'foo'
        question.answers = {
          'No'                            => '0',
          'Yes'                           => '1',
        }