如何在Angular中展示从后台到前端的错误验证

app.post('/api/submit', (req, res) => {
  // 后台验证逻辑
  if (req.body.username !== 'admin') {
    res.status(400).json({ error: '用户名错误' });
  } else if (req.body.password !== 'password') {
    res.status(400).json({ error: '密码错误' });
  } else {
    // 验证通过
    res.json({ success: true });
  }
});

<form (ngSubmit)="submitForm()" #myForm="ngForm">
  <div>
    <label for="username">用户名</label>
    <input type="text" id="username" name="username" required [(ngModel)]="formData.username">
    <div *ngIf="myForm.controls.username.invalid && (myForm.controls.username.dirty || myForm.controls.username.touched)">
      <div *ngIf="myForm.controls.username.errors.required">用户名不能为空</div>
      <div *ngIf="myForm.controls.username.errors.backendError">{{ myForm.controls.username.errors.backendError }}</div>
    </div>
  </div>
  <div>
    <label for="password">密码</label>
    <input type="password" id="password" name="password" required [(ngModel)]="formData.password">
    <div *ngIf="myForm.controls.password.invalid && (myForm.controls.password.dirty || myForm.controls.password.touched)">
      <div *ngIf="myForm.controls.password.errors.required">密码不能为空</div>
      <div *ngIf="myForm.controls.password.errors.backendError">{{ myForm.controls.password.errors.backendError }}</div>
    </div>
  </div>
  <button type="submit">提交</button>
</form>

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Component({
  selector: 'app-form',
  templateUrl: './form.component.html',
  styleUrls: ['./form.component.css']
})
export class FormComponent {
  formData = {
    username: '',
    password: ''
  };

  constructor(private http: HttpClient) {}

  submitForm() {
    this.http.post('/api/submit', this.formData).subscribe(
      (response: any) => {
        console.log(response);
      },
      (error: any) => {
        if (error.status === 400) {
          if (error.error.error === '用户名错误') {
            this.myForm.controls.username.setErrors({ backendError: '用户名错误' });
          } else if (error.error.error === '密码错误') {
            this.myForm.controls.password.setErrors({ backendError: '密码错误' });
          }
        }
      }
    );
  }
}