code: function error(utils) {
    const target = this._obj;
    isValidation(target);
    const error = target.error || null,
        json = JSON.stringify(target, null, '\t');
    this.assert(null !== error,
        '#{this} should have error but does not: ' + json,
        '#{this} should not not have error but does: ' + json
    );
    utils.flag(this, 'object', error);
  }
docs: Assert that target contains one or more errors (unsuccessful validation).
  Mutates current chainable object to be target.error.
  @example
  expect(target).to.[not].have.an.error
  target.should.[not].have.an.error