Temporary fix for workitem: 30883 - 4.8 beta: Not possible to re-enable disabled users

Added "error" class to the 'sectionValidator' control.

src/umbraco.presentation/umbraco/users/EditUser.aspx.cs

@@ -178,6 +178,7 @@ namespace umbraco.cms.presentation.user
             sectionValidator.ServerValidate += new ServerValidateEventHandler(sectionValidator_ServerValidate);
             sectionValidator.ControlToValidate = lapps.ID;
             sectionValidator.ErrorMessage = ui.Text("errorHandling", "errorMandatoryWithoutTab", ui.Text("user", "modules", base.getUser()), base.getUser());
+            sectionValidator.CssClass = "error";
 
             setupForm();
             setupChannel();