Gracefully handle errosr when cancelling stoploss orders

fixes #1933

freqtrade/freqtradebot.py

@@ -691,13 +691,22 @@ class FreqtradeBot(object):
                 # cancelling the current stoploss on exchange first
                 logger.info('Trailing stoploss: cancelling current stoploss on exchange (id:{%s})'
                             'in order to add another one ...', order['id'])
-                if self.exchange.cancel_order(order['id'], trade.pair):
+                try:
+                    self.exchange.cancel_order(order['id'], trade.pair)
+                except InvalidOrderException:
+                    logger.exception(f"Could not cancel stoploss order {order['id']} "
+                                     f"for pair {trade.pair}")
+
+                try:
                     # creating the new one
                     stoploss_order_id = self.exchange.stoploss_limit(
                         pair=trade.pair, amount=trade.amount,
                         stop_price=trade.stop_loss, rate=trade.stop_loss * 0.99
                     )['id']
                     trade.stoploss_order_id = str(stoploss_order_id)
+                except DependencyException:
+                    logger.exception(f"Could create trailing stoploss order "
+                                     f"for pair {trade.pair}.")
 
     def check_sell(self, trade: Trade, sell_rate: float, buy: bool, sell: bool) -> bool:
         if self.edge: