Handle invalid session errors for public pages in a nicer manner

Right now if the session disappears under us we spit out a ugly fatal error message. For public pages (event registration and online contribution forms), we will redirect the user back to the main entry point and start over again.

New message is:
'Your browser session has expired and we are unable to complete your form submission. We have returned you to the initial step so you can complete and resubmit the form. If you experience continued difficulties, please contact us for assistance.'