generate_db_helpers: fix crash on 'db'-less fields

tools/generate_db_helpers/generate_db_helpers.go

@@ -260,6 +260,10 @@
 
 	tag, err := tags.Get(name)
 	if err != nil {
+		if err.Error() == "tag does not exist" {
+			return "", nil, os.ErrNotExist
+		}
+
 		return "", nil, err
 	}