This was considered good practice in the days before always-on https,
but is considered superfluous today. It even gets in the way of IPv6
privacy extensions, which is the main argument for removing them today.
Previously, a NotAllowedException would be thrown if an invalid session
was encountered. However, these exceptions were not caught, and hence
would yield a fatal uncaught exception error.
At this point in Kabuki, the ErrorHandler class has not been registered yet
for error handling purposes. This error is therefore not visible if the PHP
ini directive 'display_errors' is set to 'Off'. As this is the default
production value, the script would fail with a blank page in such cases.
