title: Show errors in Angular2 forms if form items empty, but user submit form
body: <p>I have a <code>userForm</code> group which has keys <code>name</code>, <code>email</code> and <code>phone</code>. Also I have a <code>onValueChanged</code> function which one subscribes to form changes and validate data. </p>
  
  <pre><code> buildForm(): void {
      this.userForm = this.fb.group({
        'name': ['', [
            Validators.required,
          ]
        ],
        'email': [''],
        'phone':    ['', Validators.required]
      });
  
  this.userForm.valueChanges
    .subscribe(data =&gt; this.onValueChanged(data));
  </code></pre>
  
  <p>Also I have a submit button, but if user doesn't print any data and submit form my errors doesn't show. How can I fix it?</p>
  
  <pre><code>onSubmit() {
      this.submitted = true;
  }
  </code></pre>
  
  <p>This is my errors:</p>
  
  <pre><code>formErrors = {
      'name': '',
      // ...
    };
  
    validationMessages = {
      'name': {
        'required':      'Name is required.',
        'minlength':     'Name must be at least 4 characters long.',
        // ...
      },
      // ...
    };
  }
  </code></pre>
  
  <p>My <code>onValueChanged</code> function: </p>
  
  <pre><code> onValueChanged(data?: any): void {
      if (!this.registrationForm) return;
      const form = this.registrationForm;
  
      for (const field in this.formErrors) {
        this.formErrors[field] = '';
        const control: any = form.controls[field];
  
        if (control &amp;&amp; control.dirty &amp;&amp; !control.valid) {
          const messages: string = this.validationMessages[field];
          for (const key in control.errors) {
            this.formErrors[field] += messages[key] + ' ';
          }
        }
      }
    }
  </code></pre>
  
  <p>I use my validation in template like this:</p>
  
  <pre><code>&lt;div [ngClass]="formErrors.email ? 'has-danger' : '' "&gt;
      &lt;label for="email"&gt;Email &lt;span class="text-danger"&gt;*&lt;/span&gt;&lt;/label&gt;
      &lt;input type="email" formControlName="email"  id="email"&gt;
      &lt;div class="form-control-feedback"&gt;{{formErrors.email}}&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>