Fix (unreported) broken python resgistrable classes checks logic.

Logic for registering and checking properties of registrable classes was
broken, allowing to ignore some errors.

Recent fix rBeb798de101a `broke` the result of the
pyapi_idprop_datablock test, because previously that test would fail
(i.e. suceed, as it is an 'expected to break test') for a reason it was
not designed to check.

This is the problem with that kind of tests - you cannot really check
that they are failing on the expected reason(s)...

source/blender/python/intern/bpy_rna.c

@@ -7818,7 +7818,7 @@ static int pyrna_deferred_register_props(StructRNA *srna, PyObject *class_dict)
     }
   }
 
-  {
+  if (ret == 0) {
     /* This block can be removed once 2.8x is released and annotations are in use. */
     bool has_warning = false;
     while (PyDict_Next(class_dict, &pos, &key, &item)) {