WIP: preserve Issue #479 changes

backend/app/controllers/materials_controller.rb

@@ -536,7 +536,7 @@ class MaterialsController < ApplicationController
                    errors: { },
                    base_errors: ['ZIP export に必要な素材ファイルが欠損しています．'],
                    missing_files: },
-           status: :unprocessable_entity
+           status: :unprocessable_content
   end
 
   def build_uploaded_material_blob! file, file_sha256