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.
