Add help if the config is invalid

client/src/app/+admin/config/edit-custom-config/edit-custom-config.component.html

@@ -272,4 +272,5 @@ Check this checkbox, save the configuration and test with a video URL of your in
   </tabset>
 
   <input (click)="formValidated()" type="submit" i18n-value value="Update configuration" [disabled]="!form.valid">
+  <span class="form-error" i18n *ngIf="!form.valid">It seems the configration is invalid. Please search potential errors in the different tabs.</span>
 </form>

client/src/app/+admin/config/edit-custom-config/edit-custom-config.component.scss

@@ -19,6 +19,11 @@ input[type=submit] {
   @include orange-button;
 
   margin-top: 20px;
+
+  & + .form-error {
+    display: inline;
+    margin-left: 5px;
+  }
 }
 
 .inner-form-title {