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 or if we tried to load a project that was of another type.

I think we've overcomplicated this by using the ProjectFinder and loading by locale. API requests from Scratch won't have a locale set so it's confusing to make it look like we're supporting multiple locales from Scratch.

Instead use the Rails Finders for loading, which will raise as expected if a record isn't found.

Author: zetter-rpf

app/controllers/api/scratch/projects_controller.rb

@@ -62,13 +62,7 @@ def create_params
       end
 
       def load_original_project(identifier)
-        project_loader = ProjectLoader.new(identifier, [params[:locale]])
-        original_project = project_loader.load
-
-        raise ActiveRecord::RecordNotFound, I18n.t('errors.project.not_found') unless original_project
-        raise ActiveRecord::RecordNotFound, I18n.t('errors.project.not_found') unless original_project.scratch_project?
-
-        original_project
+        Project.find_by!(identifier:, project_type: Project::Types::CODE_EDITOR_SCRATCH)
       end
 
       def scratch_content_params

app/controllers/api/scratch/scratch_controller.rb

@@ -18,8 +18,7 @@ def check_scratch_feature
       end
 
       def load_project
-        project_loader = ProjectLoader.new(params[:id], [params[:locale]])
-        @project = project_loader.load
+        @project = Project.find_by!(identifier: params[:id], project_type: Project::Types::CODE_EDITOR_SCRATCH)
       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