Put back server picker for registration

* Also fix bug where you couldn't picxk a different server if
   you were already registered as a guest (because it still sent
   the access token which the new server rejected)
 * Propagate errors from UI auth back to registration so it goes
   back to the registration screen

src/components/structures/InteractiveAuth.js

@@ -91,6 +91,7 @@ export default React.createClass({
         this._authLogic.attemptAuth().then((result) => {
             this.props.onFinished(true, result);
         }).catch((error) => {
+            this.props.onFinished(false, error);
             console.error("Error during user-interactive auth:", error);
             if (this._unmounted) {
                 return;