Retrieve http response body from .net core API in angular 7

I have .net core 2.2 web api action which returns

 return BadRequest(registerResult);

registerResult is IdentityResult type object.

This api is accessed with

return this.http.post(`${environment.apiUrl}/auth/register`, user);

from angular service.

I also have this, as http interceptor:

intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    return next.handle(request).pipe(catchError(err => {
        if (err.status === 401) {
            // auto logout if 401 response returned from api
            this.authenticationService.logout();
            location.reload(true);
        }
        const error = err.error.message || err.statusText;
        return throwError(error);
    }));
}

Question is, how to return proper error messages from registerResult.Errors array, instead of "Bad request" or "Unknown error" which is currently returned?