Glib::Error::register_init(): Call Glib::wrap_register_init()

* glib/glibmm/error.cc: Call wrap_register_init() before wrap_init().
It was not necessary, when Error::register_init() was called from Glib::init(),
but Error::register_init() can be called from other places (Glib::thread_init()
and Glib::Error::throw_exception()).
* glib/glibmm/init.cc: Not necessary to call Glib::wrap_register_init() before
calling Glib::Error::register_init(). Bug #743466 (Mike Fleetwood).