psql -L log errors

Hello!

There is, in documentation of psql client (http://www.postgresql.org/docs/9.4/static/app-psql.html):
--log-file=filename
Write all query output into file filename, in addition to the normal output destination.

But it seems, that psql logs only the query output (and input if -e or -a),
but does not write the error output. How to also log the errors, please?

Thanks
Hans