code: function error(result, message, expected, actual) {
    var err = new AssertionError('`' + result.cmd + '`: ' + message);
    err.result = result;
    if (expected) err.expected = expected;
    if (actual) err.actual = actual;
    return err;
  }
docs: Create and return a new `AssertionError`.
  It will assign the given `result` to it, it will also prepend the executed command
  to the error message.
  
  Assertion error is a constructor for test and validation frameworks that implements
  standardized Assertion Error specification.
  
  For more info go visit https://github.com/chaijs/assertion-error
  
  @param {Result} result
  @param {String} error message
  @returns {AssertionError}
  @api private