seq: check whether method argument for GError is available

seq: check whether method argument for GError is available

In Rules for use of GError, the check of arguments for GError may be
preferable to avoid to unexpected function call.

This commit adds the check.

Reference: https://developer.gnome.org/glib/stable/glib-Error-Reporting.html
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>