This PR is a fix to issue #219.

Sometimes when there are insufficient funds, bot gets killed by by throwing an ‘OperationalException’ and has to be restarted.

According to me, having insufficient funds cannot be a fatal error causing application to break.

freqtrade/exchange/bittrex.py

@@ -50,6 +50,7 @@ class Bittrex(Exchange):
         temp_error_messages = [
             'NO_API_RESPONSE',
             'MIN_TRADE_REQUIREMENT_NOT_MET',
+            'INSUFFICIENT_FUNDS'
         ]
         if response['message'] in temp_error_messages:
             raise ContentDecodingError(response['message'])