Move invalid session error to login page

Summary:
With invalid session (which happens for me when I change production and dev db but can of course happen in other cases), Phabricator displays an ugly unhandled exception dialog suggesting to logging in again.
But there's no login dialog on that page.

This also changes how users with invalid session are treated on pages not requiring logging.
Previously, an exception was thrown on them. Now they are treated as unlogged users.

Test Plan: Corrupt session, go to /, login.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2236