The reason is that the GuardAuthenticationProvider *must* respond to *all* tokens
created by the system - both "pre auth" and "post auth" tokens. The reason is that
if a "post auth" token becomes not authenticated (e.g. because the user changes between
requests), then it may be passed to the provider system. If no providers respond (which
was the case before this commit), then AuthenticationProviderManager throws an exception.
The next commit will properly handle these "post auth" + "no-longer-authenticated" tokens,
which should cause a log out.
