Users sometimes logged out silently

[cbogart]

Error message says:

Error
Internal Server Error:org.springframework.security.authentication.AnonymousAuthenticationToken
cannot be cast to edu.cmu.cs.lti.discoursedb.api.browsing.controller.SystemUserAuthentication

Logging out and back in to Google authentication clears this up. User's logged-in-ness is not reflected in the user interface; they can still see stuff cached from before, and so don't know they're logged out.

Fix: Give consistent errors for bad login, and also allow a proactive check for current login status. Browser project should check this and change state.