Improve error reporting when file upload is not configured properly

app/ServiceProvider/ClassProvider.php

@@ -4,7 +4,6 @@ namespace Kanboard\ServiceProvider;
 
 use Pimple\Container;
 use Pimple\ServiceProviderInterface;
-use Kanboard\Core\ObjectStorage\FileStorage;
 use Kanboard\Core\Paginator;
 use Kanboard\Core\Http\OAuth2;
 use Kanboard\Core\Tool;
@@ -178,10 +177,6 @@ class ClassProvider implements ServiceProviderInterface
             return new HttpClient($c);
         };
 
-        $container['objectStorage'] = function () {
-            return new FileStorage(FILES_DIR);
-        };
-
         $container['cspRules'] = array(
             'default-src' => "'self'",
             'style-src' => "'self' 'unsafe-inline'",