better error handling for currentTemplate VH

ot_templating/Classes/Page/OtPageRepository.php

@@ -94,7 +94,7 @@ class OtPageRepository extends PageRepository
     public function getCurrentTemplate() {
         $rootPageUid = $this->getCurrentSiteRootPageId();
         if (!($rootPageUid >= 0)) {
-            return '';
+            throw new \RuntimeException('Can not find any root page');
         }
 
         $rootPage = $this->getPage($rootPageUid);