Specify configuration filepath when config file is not found (googleapis#1626)

Author: gpenverne

src/Google/Client.php

@@ -867,7 +867,7 @@ public function setAuthConfig($config)
   {
     if (is_string($config)) {
       if (!file_exists($config)) {
-        throw new InvalidArgumentException('file does not exist');
+        throw new InvalidArgumentException(sprintf('file "%s" does not exist', $config));
       }
 
       $json = file_get_contents($config);