Fix fetchTickers to reject funding data Bitfinex2's 'fetchTickers' raises an exception when processing a funding ticker, e.g. 'fUSD', which is included in the 'tickers' endpoint, but is not a valid symbol in the markets array. This fails silently in JS, but raises an exception in python at: ``` market = self.markets_by_id['fUSD'] ``` [ci skip]