in src/directLine.ts [842:850]
private catchPostError(error: any) {
if (error.status === 403)
// token has expired (will fall through to return "retry")
this.expiredToken();
else if (error.status >= 400 && error.status < 500)
// more unrecoverable errors
return Observable.throw(error, this.services.scheduler);
return Observable.of("retry", this.services.scheduler);
}