%env --expect-error and papermill

[BoPeng]

Papermill uses error iopub messages to decide if an error has happened. Although

%env --expect-error

changes

execute_reply = {'status': 'error'... }

to

execute_reply = {'status': 'ok'... }

Papermill will still stop because of the error message send to iopub by the kernels.

That is to say we will have to change error message to stream messages with name=stderr to make papermill work.