app.rb

not_found do
  'Bonk'
end

error(ActiveRecord::RecordNotFound) do
  status(404)
  'Aside not found'
end

new_spec.rb

  it 'allows changing the response status from the error handler' do
    mock_app { 
      
      not_found do
        'I know exactly what that is!'
      end
      
      error FooNotFound do
        status(404)
        'I have no idea what that is'
      end
      
      get '/' do
        raise FooNotFound
      end
    }
    
    get '/'
    assert_equal 404, status
    assert_equal 'I have no idea what that is', body
  end