By Marc Jones LONDON (Reuters) – The cost of Russia’s invasion of Ukraine will become a lot clearer next week, with a previously unthinkable sovereign default looming, more emergency central bank measures likely and a stock market crash guaranteed if it reopens. Moscow’s “special operation” in its former Soviet neighbour has cut Russia off from key parts of the global financial markets by the West, triggering its worst economic crisis since the 1991 fall of the Soviet Union. Wednesday could mark another low. The government is due to pay $117 million on two of its dollar-denominated bonds. But …