patch 8.1.1586: error number used in two places

Problem:    Error number used in two places.
Solution:   Renumber one. (Ken Takata)
diff --git a/src/popupwin.c b/src/popupwin.c
index f6da71d..16ec1a2 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -892,7 +892,7 @@
 	tp = find_tabpage(tabnr);
 	if (tp == NULL)
 	{
-	    semsg(_("E996: Tabpage not found: %d"), tabnr);
+	    semsg(_("E997: Tabpage not found: %d"), tabnr);
 	    return NULL;
 	}
     }