By Vitalii Hnidyi and Pavel Polityuk TROSTYANETS/LVIV, Ukraine (Reuters) – President Vladimir Putin threatened on Thursday to halt contracts supplying Europe with a third of its gas unless they are paid in Russian currency, his strongest economic riposte so far to crushing Western sanctions over his invasion of Ukraine. European governments rejected Putin’s ultimatum for Friday, with the continent’s biggest recipient of Russian gas, Germany, calling it “blackmail”. Moscow did, however, offer a mechanism for buyers to obtain roubles via a Russian bank. The energy showdown has huge ramifications…