LONDON, Sept 8( Reuters)-Moody's next move on Gabon's credit rating following the country's military coup this month will depend on its economic impact, while it is also working through another downgrade review of Niger following its coup there in late July. The two countries have seen West and Central Africa's seventh and eighth coups in the last three years, and while Niger's Caa2 rating has already been cut by two full notches by Moody's, the firm has maintained a" stable" outlook on its slig