raise exception when no functional categories available yet

backend/metagenedb/apps/catalog/management/commands/create_update_eggnog.py

@@ -24,6 +24,8 @@ class ImportEggNog(object):
 
     def _build_functional_category_dict(self):
         all_categories = EggNogFunctionalCategory.objects.all()
+        if not all_categories:
+            raise Exception("You need to create Functional categories first.")
         self.functional_cat = {cat.category_id: cat for cat in all_categories}
 
     def link_functional_category(self, eggnog_dict):