By Tommy Wilkes, Sujata Rao and Gertrude Chavez-Dreyfuss NEW YORK/LONDON (Reuters) -Cryptocurrencies that seemed to be defying gravity just weeks ago came back down to earth with a bump on Wednesday after a roller-coaster ride which could undermine their potential as mainstream investments. The two main digital currencies, bitcoin and ether, fell as much as 30% and 45% respectively, but significantly pared losses after two of their biggest backers - Tesla Inc chief Elon Musk and Ark Invest's Chief Executive Cathie Wood - indicated their support for bitcoin. While many analysts thought the...