Fix to allow FacebookRedirectLoginHelper->getAccessToken() to throw

Don't know why the exception was being caught for `FacebookRedirectLoginHelper->getAccessToken()`, but in the case that the access token could not be obtained, the method would just return `null` with no way of finding out what went wrong. (In my case: `Couldn't resolve host 'graph.facebook.com'`). So this fixes that and allows the method to throw.