py: Use mp_raise_msg helper function where appropriate.

Saves the following number of bytes of code space: 176 for bare-arm, 352
for minimal, 272 for unix x86-64, 140 for stmhal, 120 for esp8266.
diff --git a/py/objnamedtuple.c b/py/objnamedtuple.c
index 38cda1a..18931a1 100644
--- a/py/objnamedtuple.c
+++ b/py/objnamedtuple.c
@@ -73,7 +73,7 @@
     } else {
         // delete/store attribute
         // provide more detailed error message than we'd get by just returning
-        nlr_raise(mp_obj_new_exception_msg(&mp_type_AttributeError, "can't set attribute"));
+        mp_raise_msg(&mp_type_AttributeError, "can't set attribute");
     }
 }