Fix non existing function called

core/API/ResponseBuilder.php

@@ -154,7 +154,7 @@ class ResponseBuilder
         // If we are in tests, show full backtrace
         if (defined('PIWIK_PATH_TEST_TO_ROOT')) {
             if (self::DISPLAY_BACKTRACE_DEBUG
-                || Piwik_ShouldPrintBackTraceWithMessage()
+                || \Piwik_ShouldPrintBackTraceWithMessage()
             ) {
                 $message = $e->getMessage() . " in \n " . $e->getFile() . ":" . $e->getLine() . " \n " . $e->getTraceAsString();
             } else {

core/DataTable/Renderer.php

@@ -125,7 +125,7 @@ abstract class Renderer
     protected function getExceptionMessage()
     {
         $message = $this->exception->getMessage();
-        if (Piwik_ShouldPrintBackTraceWithMessage()) {
+        if (\Piwik_ShouldPrintBackTraceWithMessage()) {
             $message .= "\n" . $this->exception->getTraceAsString();
         }
         return self::renderHtmlEntities($message);