function errorDiff(actual, expected) {
    return diff
      .diffWordsWithSpace(actual, expected)
      .map(function (str) {
        if (str.added) {
          return colorLines('diff added inline', str.value);
        }
        if (str.removed) {
          return colorLines('diff removed inline', str.value);
        }
        return str.value;
      })
      .join('');
  }
Returns character diff for `err`.

@private
@param {String} actual
@param {String} expected
@return {string} the diff