title: input field or help text doesn't turn red when field is invalid
body: <p>I used to implement an Angular 2/4 application with Bootstrap 3 and used the Reactive Forms approach. I had a field-validation where the border of the input-field turned red and an error message appeared under the field in red font color.</p>
  
  <p>it looks like this:</p>
  
  <pre><code>&lt;div class="form-group row"
                  [ngClass]="{'has-error':    (sourcesForm.get('sourceName').touched || 
                                              sourcesForm.get('sourceName').dirty) &amp;&amp; 
                                              !sourcesForm.get('sourceName').valid }"&gt;
                  &lt;label class="col-md-2 col-form-label"
                          for="sourceNameId"&gt;Source Name&lt;/label&gt;
                  &lt;div class="col-md-8"&gt;
                      &lt;input  class="form-control"
                              id="sourceNameId"
                              type="text"
                              placeholder="Source Name (required)"
                              formControlName="sourceName" /&gt;
                      &lt;span class="help-block" *ngIf="(sourcesForm.get('sourceName').touched || 
                                                      sourcesForm.get('sourceName').dirty) &amp;&amp; 
                                                      sourcesForm.get('sourceName').errors"&gt;
                          &lt;span *ngIf="sourcesForm.get('sourceName').errors.required"&gt;
                              Please enter the Source Name.
                          &lt;/span&gt;
                          &lt;span *ngIf="sourcesForm.get('sourceName').errors.minlength"&gt;
                              The Source Name must be longer than 3 characters.
                          &lt;/span&gt;
                          &lt;span *ngIf="sourcesForm.get('sourceName').errors.maxlength"&gt;
                              The Source Name is too long.
                          &lt;/span&gt;
                      &lt;/span&gt;
                  &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Now i have to use Bootstrap 4 and neither the error message or the input-field turns red. How do i realise this? I tried to change the class of the parent span-block to "form-text" but it didn't work.</p>