Bug 1579999: Class 'Session' not found in errors.php

The warning is
PHP Fatal error:  Class 'Session' not found in htdocs/lib/errors.php on line 128

behatnotneeded

Change-Id: I8ebbc6d427d59e7a3515b09ee5b1f67c14957122
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit 84821546)