added fix for insufficient funds error from bitrex, looking at possibility to test it it

freqtrade/exchange/bittrex.py

@@ -5,7 +5,7 @@ from bittrex.bittrex import API_V1_1, API_V2_0
 from bittrex.bittrex import Bittrex as _Bittrex
 from requests.exceptions import ContentDecodingError
 
-from freqtrade import OperationalException
+from freqtrade import OperationalException, NotEnoughFundsExeption
 from freqtrade.exchange.interface import Exchange
 
 logger = logging.getLogger(__name__)
@@ -63,11 +63,19 @@ class Bittrex(Exchange):
         data = _API.buy_limit(pair.replace('_', '-'), amount, rate)
         if not data['success']:
             Bittrex._validate_response(data)
-            raise OperationalException('{message} params=({pair}, {rate}, {amount})'.format(
-                message=data['message'],
-                pair=pair,
-                rate=rate,
-                amount=amount))
+
+            if data['message'] == "INSUFFICIENT_FUNDS":
+                raise NotEnoughFundsExeption('{message} params=({pair}, {rate}, {amount})'.format(
+                    message=data['message'],
+                    pair=pair,
+                    rate=rate,
+                    amount=amount))
+            else:
+                raise OperationalException('{message} params=({pair}, {rate}, {amount})'.format(
+                    message=data['message'],
+                    pair=pair,
+                    rate=rate,
+                    amount=amount))
         return data['result']['uuid']
 
     def sell(self, pair: str, rate: float, amount: float) -> str: