code: function errorDiff(actual, expected) {
    return diff
      .diffWordsWithSpace(actual, expected)
      .map(function(str) {
        if (str.added) {
          return colorLines('diff added', str.value);
        }
        if (str.removed) {
          return colorLines('diff removed', str.value);
        }
        return str.value;
      })
      .join('');
  }
docs: Returns character diff for `err`.
  
  @private
  @param {String} actual
  @param {String} expected
  @return {string} the diff