Check if the vendor path is missing and complain about it

index.php

@@ -4,6 +4,10 @@
  * Friendica
  */
 
+if (!file_exists(__DIR__ . '/vendor/autoload.php')) {
+	die('Vendor path not found. Please execute "bin/composer.phar --no-dev install" on the command line in the web root.');
+}
+
 require __DIR__ . '/vendor/autoload.php';
 
 // We assume that the index.php is called by a frontend process