extensions-tool: Add common option to silence errors

Error reporting is useful when used interactively, but often undesirable
when used in scripts. Account for that with a common --quiet option,
which is more convenient than redirection stderr to /dev/null.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391

subprojects/extensions-tool/src/command-pack.c

@@ -480,6 +480,7 @@ handle_pack (int argc, char *argv[], gboolean do_help)
   g_option_context_set_help_enabled (context, FALSE);
   g_option_context_set_summary (context, _("Create an extension bundle"));
   g_option_context_add_main_entries (context, entries, GETTEXT_PACKAGE);
+  g_option_context_add_group (context, get_option_group());
 
   if (do_help)
     {