Angular 4按顺序执行请求，一个请求失败则停止

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import { catchError } from 'rxjs/operators';

@Component({
  selector: 'app-example',
  template: `
    <button (click)="startRequests()">开始请求</button>
  `
})
export class ExampleComponent {
  constructor(private http: HttpClient) {}

  startRequests() {
    this.request1()
      .pipe(
        catchError(error => {
          console.error('请求1失败:', error);
          return Observable.throw(error);
        })
      )
      .subscribe(() => {
        this.request2()
          .pipe(
            catchError(error => {
              console.error('请求2失败:', error);
              return Observable.throw(error);
            })
          )
          .subscribe(() => {
            console.log('所有请求成功');
          });
      });
  }

  request1(): Observable<any> {
    return this.http.get('请求1的URL');
  }

  request2(): Observable<any> {
    return this.http.get('请求2的URL');
  }
}