function errorDiff (err) {
  return diff.diffWordsWithSpace(err.actual, err.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('');
}
Return a character diff for `err`.

@api private
@param {Error} err
@return {string}