Fixes #6841 - catch the exception when loading tracker environment to prevent error on screen

piwik.php

@@ -67,11 +67,15 @@ require_once PIWIK_INCLUDE_PATH . '/core/Tracker/Cache.php';
 require_once PIWIK_INCLUDE_PATH . '/core/Tracker/Request.php';
 require_once PIWIK_INCLUDE_PATH . '/core/Cookie.php';
 
-Tracker::loadTrackerEnvironment();
-
 session_cache_limiter('nocache');
 @date_default_timezone_set('UTC');
 
+try {
+    Tracker::loadTrackerEnvironment();
+} catch (Exception $e) {
+    // eg. Piwik is not installed yet
+}
+
 $tracker    = new Tracker();
 $requestSet = new RequestSet();