[ticket/11196] Changed 401 response message in session.php.

In session.php 401 response message was "Not Authorized".
I changed it to "Unauthorized".

PHPBB3-11196
Merge branch 'ticket/erangamapa/11196' into develop-olympus

* ticket/erangamapa/11196:
  [ticket/11196] Changed 401 response message in session.php.