Workflow to ensure correct catch of Omniauth login failure. Specs included.

omniauth.rb

OmniAuth.config.on_failure = Proc.new { |env|
  OmniAuth::FailureEndpoint.new(env).redirect_to_failure
}

routes.rb

  get 'auth/failure',            to: 'sessions#failure'

sessions_controller.rb

  def failure
    redirect_to root_path, aler: "There was an error while authenticating"
  end

sign_in_spec.rb

feature "Sign in with Facebook" do

  context "with invalid data" do
    scenario "doesn't create an user" do
      OmniAuth.config.mock_auth[:facebook] = fb_invalid_data

      visit root_path
      click_on "Connect"

      expect(User.count).to be_zero
    end

    scenario "redirects if a failure is found" do
      OmniAuth.config.mock_auth[:facebook] = :invalid_credentials

      visit root_path
      click_on "Connect"

      expect(current_path).to eql root_path
    end
  end
end