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/objarray.c b/py/objarray.c
index 2cd0fef..8e1d32f 100644
--- a/py/objarray.c
+++ b/py/objarray.c
@@ -95,7 +95,7 @@
 STATIC mp_obj_array_t *array_new(char typecode, mp_uint_t n) {
     int typecode_size = mp_binary_get_size('@', typecode, NULL);
     if (typecode_size == 0) {
-        nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "bad typecode"));
+        mp_raise_msg(&mp_type_ValueError, "bad typecode");
     }
     mp_obj_array_t *o = m_new_obj(mp_obj_array_t);
     #if MICROPY_PY_BUILTINS_BYTEARRAY && MICROPY_PY_ARRAY
@@ -395,7 +395,7 @@
                     mp_obj_array_t *src_slice = MP_OBJ_TO_PTR(value);
                     if (item_sz != mp_binary_get_size('@', src_slice->typecode & TYPECODE_MASK, NULL)) {
                     compat_error:
-                        nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "lhs and rhs should be compatible"));
+                        mp_raise_msg(&mp_type_ValueError, "lhs and rhs should be compatible");
                     }
                     src_len = src_slice->len;
                     src_items = src_slice->items;