Return a 404 if a Scratch project doesn't exist

Fixes RaspberryPiFoundation/digital-editor-issues#1269

Previously a 500 was raised because we would try to access the scratch component of an `nil` object.

I've also added another case for if the project exists but isn't a scratch project as I expect that would error too.

Author: zetter-rpf

app/controllers/api/scratch/scratch_controller.rb

@@ -20,6 +20,9 @@ def check_scratch_feature
       def load_project
         project_loader = ProjectLoader.new(params[:id], [params[:locale]])
         @project = project_loader.load
+
+        raise ActiveRecord::RecordNotFound, I18n.t('errors.project.not_found') unless @project
+        raise ActiveRecord::RecordNotFound, I18n.t('errors.project.not_found') unless @project.scratch_project?
       end
     end
   end

spec/features/scratch/showing_a_scratch_project_spec.rb

@@ -18,4 +18,18 @@
     data = JSON.parse(response.body, symbolize_names: true)
     expect(data).to have_key(:targets)
   end
+
+  it 'returns a 404 if project does not exist' do
+    get '/api/scratch/projects/non_existent_project'
+
+    expect(response).to have_http_status(:not_found)
+  end
+
+  it 'returns a 404 if project is not a scratch project' do
+    project = create(:project, project_type: Project::Types::PYTHON, locale: 'en')
+
+    get "/api/scratch/projects/#{project.identifier}"
+
+    expect(response).to have_http_status(:not_found)
+  end
 end