Fix img proxy file get content

When getting a resource, check if file get content returns false, else return a placeholder or a 404.

Error caught :

{"message":"Warning: file_get_contents(https://imgproxy.agkit.io/GHWkcl_MTJB-nKTSuKUb4qSg030a7VbsIit7fCfWtHw/rs:fit:20:20/g:ce/el:1/aHR0cHM6Ly9zaWducy5zMy5mci1wYXIuc2N3LmNsb3VkL0ludGVybWFyY2hlX1N1cGVyX2V0X0RyaXZlLnBuZw.png): failed to open stream: HTTP request failed! HTTP/1.0 404 Not Found\r\n","context":{"exception":{"class":"ErrorException","message":"Warning: file_get_contents(https://imgproxy.agkit.io/GHWkcl_MTJB-nKTSuKUb4qSg030a7VbsIit7fCfWtHw/rs:fit:20:20/g:ce/el:1/aHR0cHM6Ly9zaWducy5zMy5mci1wYXIuc2N3LmNsb3VkL0ludGVybWFyY2hlX1N1cGVyX2V0X0RyaXZlLnBuZw.png): failed to open stream: HTTP request failed! HTTP/1.0 404 Not Found\r\n","code":0,"file":"/app/vendor/aboutgoods/konnect-bundle/Controller/ResourceController.php:39"}},"level":300,"level_name":"WARNING","channel":"php","datetime":{"date":"2020-06-11 11:35:06.765700","timezone_type":3,"timezone":"UTC"},"extra":[]}