py/runtime: Be sure that non-intercepted thrown object is an exception.

The VM expects that, if mp_resume() returns MP_VM_RETURN_EXCEPTION, then
the returned value is an exception instance (eg to add a traceback to it).
It's possible that a value passed to a generator's throw() is not an
exception so must be explicitly checked for if the thrown value is not
intercepted by the generator.

Thanks to @jepler for finding the bug.

py/runtime.c

@@ -1282,7 +1282,7 @@ mp_vm_return_kind_t mp_resume(mp_obj_t self_in, mp_obj_t send_value, mp_obj_t th
         // will be propagated up. This behavior is approved by test_pep380.py
         // test_delegation_of_close_to_non_generator(),
         //  test_delegating_throw_to_non_generator()
-        *ret_val = throw_value;
+        *ret_val = mp_make_raise_obj(throw_value);
         return MP_VM_RETURN_EXCEPTION;
     }
 }

tests/basics/gen_yield_from_throw3.py

@@ -28,3 +28,30 @@ print(g.throw(123))
 g = gen()
 print(next(g))
 print(g.throw(ZeroDivisionError))
+
+# this user-defined generator doesn't have a throw() method
+class Iter2:
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        return 1
+
+def gen2():
+    yield from Iter2()
+
+# the thrown ValueError is not intercepted by the user class
+g = gen2()
+print(next(g))
+try:
+    g.throw(ValueError)
+except:
+    print('ValueError')
+
+# the thrown 123 is not an exception so raises a TypeError
+g = gen2()
+print(next(g))
+try:
+    g.throw(123)
+except TypeError:
+    print('TypeError')