Python: improved error handling if response object isn't iterable.

According to the documentation, PyObject_GetIter():

 | Raises TypeError and returns NULL if the object cannot be iterated.

Previously, this exception wasn't printed or cleared and remained unhandled.

src/nxt_python_wsgi.c

@@ -425,6 +425,7 @@ nxt_python_request_handler(nxt_unit_request_info_t *req)
         if (nxt_slow_path(iterator == NULL)) {
             nxt_unit_req_error(req, "the application returned "
                                "not an iterable object");
+            PyErr_Print();
 
             goto fail;
         }