patch 8.2.4003: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/optionstr.c b/src/optionstr.c
index f555bee..f4a8cb8 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -1003,9 +1003,9 @@
 #if defined(FEAT_GUI_GTK)
 	if (errmsg == NULL && varp == &p_tenc && gui.in_use)
 	{
-	    // GTK+ 2 uses only a single encoding, and that is UTF-8.
+	    // GTK uses only a single encoding, and that is UTF-8.
 	    if (STRCMP(p_tenc, "utf-8") != 0)
-		errmsg = N_("E617: Cannot be changed in the GTK+ 2 GUI");
+		errmsg = e_cannot_be_changed_in_gtk_GUI;
 	}
 #endif