Add test coverage for raising/rescuing of InvalidNode

I hope this might help avoid a repeat of #2474 in which the implementation of
`Outcome#transition` was accidentally removed [1].

I've also added test coverage for the rescuing of `FlowRegistry::NotFound` while
I was at it.

[1]: 0929036

Author: floehopper

test/integration/smart_answers_controller_error_handling_test.rb

@@ -0,0 +1,37 @@
+require_relative '../integration_test_helper'
+require 'gds_api/test_helpers/content_api'
+
+class SmartAnswersControllerErrorHandlingTest < ActionDispatch::IntegrationTest
+  include GdsApi::TestHelpers::ContentApi
+
+  setup do
+    @flow_registry = stub('flow-registry')
+    SmartAnswer::FlowRegistry.stubs(:instance).returns(@flow_registry)
+    stub_content_api_default_artefact
+  end
+
+  context 'when SmartAnswer::InvalidNode raised' do
+    setup do
+      flow = stub('flow', name: 'flow-name')
+      flow.stubs(:process).raises(SmartAnswer::InvalidNode)
+      @flow_registry.stubs(:find).returns(flow)
+    end
+
+    should 'set response status code to 404' do
+      get '/flow-name'
+      assert_response 404
+    end
+  end
+
+  context 'when SmartAnswer::InvalidNode raised' do
+    setup do
+      flow = stub('flow', name: 'flow-name')
+      @flow_registry.stubs(:find).raises(SmartAnswer::FlowRegistry::NotFound)
+    end
+
+    should 'set response status code to 404' do
+      get '/flow-name'
+      assert_response 404
+    end
+  end
+end

test/unit/outcome_test.rb

@@ -0,0 +1,17 @@
+require_relative '../test_helper'
+
+module SmartAnswer
+  class OutcomeTest < ActiveSupport::TestCase
+    setup do
+      @outcome = Outcome.new(nil, 'node-name')
+    end
+
+    context '#transition' do
+      should 'raise InvalidNode exception so app responds with 404 Not Found' do
+        assert_raises(InvalidNode) do
+          @outcome.transition
+        end
+      end
+    end
+  end
+end