fftools: log unconnected filter output label

fftools/ffmpeg_filter.c

@@ -1410,7 +1410,9 @@ int fg_finalise_bindings(void)
             OutputFilter *output = fg->outputs[j];
             if (!output->bound) {
                 av_log(fg, AV_LOG_FATAL,
-                       "Filter %s has an unconnected output\n", output->name);
+                       "Filter '%s' has output %d (%s) unconnected\n",
+                       output->name, j,
+                       output->linklabel ? (const char *)output->linklabel : "unlabeled");
                 return AVERROR(EINVAL);
             }
         }