How to perform error handling from multiple Observables in Angular 2 / Ionic 3?

I am trying to cache some data using multiple Observables in an Ionic 3 application:

import { Observable } from "rxjs/Observable";
import 'rxjs/add/observable/forkJoin'

// saves data for a single article
private saveToCache(articleId: number): Observable<any> {
    this.loadingCtrl.create({ content: "Caching data. Please wait..." });
    this.loadingCtrl.present();

    let key = articleId;
    let obs = this.articleService.getArticleFullData(key);
    obs.subscribe(data => {
        this.loadingCtrl.dismiss();
        this.cache.saveItem("" + key, data as ArticleFullData);
        this.loggingService.logInfo("Cache article: " + key);
    },
    err => {
        this.loggingService.logError("Failed to cache data: " + key);
        this.loadingCtrl.dismiss();

        var toastObj = this.toastCtrl.create({ message: "Failed to cache data: ", duration: 2000 });
        toastObj.present();
    }
    );

    return obs;
}

// handler to perform caching for a list of articles
onCacheRefresh() {
    // articles are not loaded for some reason
    if (!this.articles)
        return;

    var obsArray = [];
    for (let article of this.articles) {
        let key = "" + article.ArticleId;

        // already in cache
        this.cache.getItem(key)
            .then(data => {
                console.log("Cache item already exists: ", data);
            })
            .catch(err => {
                obsArray.push(this.saveToCache(article.ArticleId));
            });
    }

    let err: boolean = false;
    Observable.forkJoin(obsArray).toPromise()
        .catch(err => {
            if (err)
                return;
            err = true;

            this.loggingService.logError("Failed to cache data: ", JSON.stringify(err));
            var toastObj = this.toastCtrl.create({ message: "Failed to cache data: ", duration: 2000 });
            toastObj.present();
        });
}

If, for any reason, data fetch fails the catch for the forkJoin will execute for each failure. What I want is to be able to display the toast notification only once.

Question: How should I handle errors coming from multiple Observables?