Fixed error message when serializing with required fields missing.

python/message.c

diff --git a/python/message.c b/python/message.c
index 45c70e9..0b501fc 100644
--- a/python/message.c
+++ b/python/message.c
@@ -1328,6 +1328,18 @@
 
   if (!pb) {
     PyUpb_ModuleState* state = PyUpb_ModuleState_Get();
+    PyObject* errors = PyUpb_CMessage_FindInitializationErrors(_self, NULL);
+    if (PyList_Size(errors) != 0) {
+      PyObject* comma = PyUnicode_FromString(",");
+      PyObject* missing_fields = PyUnicode_Join(comma, errors);
+      PyErr_Format(state->encode_error_class,
+                   "Message %s is missing required fields: %U",
+                   upb_msgdef_fullname(msgdef), missing_fields);
+      Py_XDECREF(comma);
+      Py_XDECREF(missing_fields);
+      Py_DECREF(errors);
+      goto done;
+    }
     PyErr_Format(state->encode_error_class, "Failed to serialize proto");
     goto done;
   }

python/pb_unit_tests/reflection_test_wrapper.py

diff --git a/python/pb_unit_tests/reflection_test_wrapper.py b/python/pb_unit_tests/reflection_test_wrapper.py
index 76e3091..f182ccd 100644
--- a/python/pb_unit_tests/reflection_test_wrapper.py
+++ b/python/pb_unit_tests/reflection_test_wrapper.py
@@ -45,8 +45,6 @@
 reflection_test.SerializationTest.testFieldDataDescriptor.__unittest_expecting_failure__ = True
 reflection_test.SerializationTest.testFieldProperties.__unittest_expecting_failure__ = True
 reflection_test.SerializationTest.testInitArgsUnknownFieldName.__unittest_expecting_failure__ = True
-reflection_test.SerializationTest.testSerializeUninitialized.__unittest_expecting_failure__ = True
-reflection_test.SerializationTest.testSerializeUninitializedSubMessage.__unittest_expecting_failure__ = True
 
 if __name__ == '__main__':
   unittest.main(module=reflection_test, verbosity=2)