dashboard: show devices error within the form

dashboard/public/views/app.html

@@ -1159,10 +1159,11 @@
           <div class="col-md-12">
             <form role="form" name="devicesForm" ng-submit="resources.submitDevices()" autocomplete="off">
               <fieldset>
-                <div class="form-group">
+                <div class="form-group" ng-class="{ 'has-error': resources.error.devices }">
                   <label class="control-label" for="devicesInput">Devices <sup><a ng-href="https://docs.cloudron.io/apps/#devices" class="help" target="_blank"><i class="fa fa-question-circle"></i></a></sup></label>
                   <p>Comma serparated list of devices mounted into the app</p>
                   <input type="text" class="form-control" id="devicesInput" ng-model="resources.devices" placeholder="/dev/ttyUSB, /dev/hidraw0, ..." ng-disabled="resources.busy"/>
+                  <span class="text-danger" ng-show="resources.error.devices">{{ resources.error.devices }}</span>
                 </div>
               </fieldset>
             </form>