Angular2 : HTTP error handling

I am trying to reproduce the following behaviour that I successfully implemented using Angular 1.x interceptors and promises, in a Angular2 based Ionic2 app :

Intercept HTTP request errors
If the status code is 401, then

a. make another request to register the client. This will provide some token that I can subsequently attach to each request.

b. Retry the orignal request and provide the result to the caller through the promise / observable
If the error status is anything other than 401 just let the flow normally to the caller

NB: The register process does not need any intervention from the user (no login) so I want it to be completely transparent for the user. I register once when the app is first loaded, but the session will eventually expire and I need to automatically re-register.

This is the original Angular 1 implementation (I include only the responseErrorpart of the interceptor):

responseError : function(rejection){
    if(rejection.status == 401){
        $log.info('Intercepting unauthorized 401 response. Will try to re-register the session and retry the request.');

        var security = $injector.get('SecurityService');
        var $http = $injector.get('$http');

        return security.registerSession().then(function(){
            return $http(rejection.config);
        });

    }else{
        return rejection;
    }
}

Now, I have wrapped Angular2's HTTP service and I can do simple things like adding a header to each request. However I am struggling to reproduce the same behaviour using Angular2 and Observables.

My attempt so far, this is the request method of my HTTP wrapper that my services will call :

request(url: string | Request, options?: RequestOptionsArgs): Observable<Response> {

    let req: Request = this.buildRequest(url, options);

    this.beforeCall(req);

    var observable = this.http
        .request(req)
        .retry(3)
        .catch(res => {
            debugger;
            console.log("ERROR! " + res.status);
            if(res.status == 401){
                this.logger.log('Unauthorized request. Trying to register the session and then retry the request.');
                return this.securityService.registerSession().subscribe(() => {
                    // What to do here so the client gets the result of the original request ??
                });
            }
        })
        .do((res:Response) => { this.afterCall(req, res) });

    return observable;
}

You could execute again the request after initialize the session using the flatMap operator of observables:

    var observable = this.http
        .request(req)
        .retry(3)
        .catch(res => {
            debugger;
            console.log("ERROR! " + res.status);
            if(res.status == 401){
                this.logger.log('Unauthorized request. Trying to register the session and then retry the request.');
                return this.securityService.registerSession().flatMap(() => {
                  return this.http.request(req); // <--------
                });
            }
        })
        .do((res:Response) => { this.afterCall(req, res) });

    return observable;
}