Exit non-zero if composer bin not found (#15120)

scripts/composer_wrapper.php

@@ -102,6 +102,7 @@ if ($exec) {
     exit($exit_code);
 } else {
     echo "Composer not available, please manually install composer.\n";
+    exit(1);
 }
 
 function curl_fetch($url, $proxy, $use_https, $output = false)