handle http errors spec.ts

loadStarred() {
  this.http.get<Repo[]>(`https://api.github.com/users/${this.userName}/starred`)
    .pipe(
      tap(
        response => {
          this.repos = response;
          this.errorMessage = null;
        }
      ),
      catchError(error => {
        this.repos = [];
        this.errorMessage = error && error.error && error.error.message || 'Unknown error';
        return of([]);
      })
    )
    .subscribe();
}