Angular2：验证表单而不在每个输入定义错误消息？

<form [ngFormModel]="form">
        <input type="text" ngControl="username" />

        <p *ngIf="username.pending">Fetching data from the server...</p>

        <div *ngIf="username.dirty && !username.valid && !username.pending">
          <p *ngIf="username.errors.required">Username is required.</p>
          <p *ngIf="username.errors.startsWithNumber">Your username can't start with a number</p>
          <p *ngIf="username.errors.usernameTaken">This username is taken</p>
        </div>

        <button (click)="submitData()" [disabled]="!form.valid" class="btn btn-primary">Sumbit data</button>
</form>

constructor(private builder: FormBuilder) {

    this.username = new Control(
        "",Validators.compose([Validators.required,UsernameValidator.startsWithNumber]),UsernameValidator.usernameTaken
    );

    this.form = builder.group({
        username:  this.username
    });
}

对于每个输入,我需要定义许多错误消息.我认为这不好.
我想像jquery.validate,我只是定义输入,错误消息将自动渲染,像这样

<input required name="username" maxlength='8' pattern="^(?!s|.*s$).*$" ...>