Improve "404.md not found" exception message

lib/Pico.php

@@ -695,7 +695,7 @@ class Pico
 
         if (!file_exists($this->getConfig('content_dir') . $errorFile)) {
             $errorFile = ($errorFileDir === '.') ? '404' . $this->getConfig('content_ext') : $errorFile;
-            throw new RuntimeException('Required "' . $errorFile . '" not found');
+            throw new RuntimeException('Required "' . $this->getConfig('content_dir') . $errorFile . '" not found');
         }
 
         return $this->loadFileContent($this->getConfig('content_dir') . $errorFile);