py/objtype: Remove comment about catching exc from user __getattr__.

Any exception raised in a user __getattr__ should be propagated out.  A
test is added to verify these semantics.

py/objtype.c

@@ -654,7 +654,6 @@ STATIC void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *des
         mp_load_method_maybe(self_in, MP_QSTR___getattr__, dest2);
         if (dest2[0] != MP_OBJ_NULL) {
             // __getattr__ exists, call it and return its result
-            // XXX if this fails to load the requested attr, should we catch the attribute error and return silently?
             dest2[2] = MP_OBJ_NEW_QSTR(attr);
             dest[0] = mp_call_method_n_kw(1, 0, dest2);
             return;

tests/basics/getattr.py

@@ -9,3 +9,20 @@ class A:
 
 a = A({'a':1, 'b':2})
 print(a.a, a.b)
+
+# test that any exception raised in __getattr__ propagates out
+class A:
+    def __getattr__(self, attr):
+        if attr == "value":
+            raise ValueError(123)
+        else:
+            raise AttributeError(456)
+a = A()
+try:
+    a.value
+except ValueError as er:
+    print(er)
+try:
+    a.attr
+except AttributeError as er:
+    print(er)