r.to.vect: print useful info before quitting

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@60953 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.to.vect/lines.c

@@ -495,7 +495,13 @@ static int update_list(int count)
 	    v_list[col - 1].right = start_line(1);
 	break;
     default:
-	G_debug(1, "Crowded cell %xH (%d,%d)", count, row, col);
+	{
+	    double x, y;
+	    x = Rast_col_to_easting((double)(col + .5), &cell_head);
+	    y = Rast_row_to_northing((double)(row + .5), &cell_head);
+	    G_message(_("Crowded cell at (%f, %f): row %d, col %d, count %d"),
+		      x, y, row, col, count);
+	}
 	G_fatal_error(_("Raster map is not thinned properly.\nPlease run r.thin."));
     }				/* switch count */