ffmpeg: clarify error message in case of bitstream filter opening failure

ffmpeg.c

@@ -988,7 +988,7 @@ static void write_frame(AVFormatContext *s, AVPacket *pkt, OutputStream *ost)
             av_free_packet(pkt);
             new_pkt.destruct = av_destruct_packet;
         } else if (a < 0) {
-            av_log(NULL, AV_LOG_ERROR, "%s failed for stream %d, codec %s",
+            av_log(NULL, AV_LOG_ERROR, "Failed to open bitstream filter %s for stream %d with codec %s",
                    bsfc->filter->name, pkt->stream_index,
                    avctx->codec ? avctx->codec->name : "copy");
             print_error("", a);