Improved Persona error handling

The client updates associated with WardCunningham/wiki#37

When the verifier fails, rather than causing an AJAX error the server will return the reason for the failure. 

The client is updated to capture the failure, display an error dialog, and log the user out of site (to prevent login failure loops).

A side effect - if somebody other than the site owner logs in, this is now handled better.

Also resolves #3 by calling ```navigator.id.logout()``` to log the user out of the site.