Show serialization exceptions in DAG parsing log (#17277)

Make sure that any exceptions that happen when writing serialized DAGs
to the db get written to the DAG parsing log, instead of only being added
to `import_errors` for consumption via the UI.

(cherry picked from commit 9cd5a97)

airflow/models/dagbag.py

@@ -595,6 +595,7 @@ def _serialize_dag_capturing_errors(dag, session):
             except OperationalError:
                 raise
             except Exception:
+                self.log.exception("Failed to write serialized DAG: %s", dag.full_filepath)
                 return [(dag.fileloc, traceback.format_exc(limit=-self.dagbag_import_error_traceback_depth))]
 
         # Retry 'DAG.bulk_write_to_db' & 'SerializedDagModel.bulk_sync_to_db' in case

tests/models/test_dagbag.py

@@ -703,7 +703,9 @@ def test_serialized_dag_errors_are_import_errors(self, mock_serialize):
             )
             assert dagbag.import_errors == {}
 
-            dagbag.sync_to_db(session=session)
+            with self.assertLogs(level="ERROR") as cm:
+                dagbag.sync_to_db(session=session)
+            self.assertIn("SerializationError", "\n".join(cm.output))
 
             assert path in dagbag.import_errors
             err = dagbag.import_errors[path]