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/modthread.c b/py/modthread.c
index c358cdf..6f55281 100644
--- a/py/modthread.c
+++ b/py/modthread.c
@@ -239,7 +239,7 @@
     } else {
         // positional and keyword arguments
         if (mp_obj_get_type(args[2]) != &mp_type_dict) {
-            nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "expecting a dict for keyword args"));
+            mp_raise_msg(&mp_type_TypeError, "expecting a dict for keyword args");
         }
         mp_map_t *map = &((mp_obj_dict_t*)MP_OBJ_TO_PTR(args[2]))->map;
         th_args = m_new_obj_var(thread_entry_args_t, mp_obj_t, pos_args_len + 2 * map->used);