patch 8.2.3986: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/popupwin.c b/src/popupwin.c
index ee33a43..a556f68 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1893,7 +1893,7 @@
 		    && !(argvars[0].v_type == VAR_LIST
 			&& argvars[0].vval.v_list != NULL))
 	{
-	    emsg(_("E450: buffer number, text or a list required"));
+	    emsg(_(e_buffer_number_text_or_list_required));
 	    return NULL;
 	}
 	if (argvars[1].v_type != VAR_DICT || argvars[1].vval.v_dict == NULL)