catch MIN_TRADE_REQUIREMENT_NOT_MET as non-critical exception (#237)

* add MIN_TRADE_REQUIREMENT_NOT_MET to response validation

* implement test

freqtrade/exchange/bittrex.py

@@ -45,8 +45,12 @@ class Bittrex(Exchange):
         Validates the given bittrex response
         and raises a ContentDecodingError if a non-fatal issue happened.
         """
-        if response['message'] == 'NO_API_RESPONSE':
-            raise ContentDecodingError('Unable to decode bittrex response')
+        temp_error_messages = [
+            'NO_API_RESPONSE',
+            'MIN_TRADE_REQUIREMENT_NOT_MET',
+        ]
+        if response['message'] in temp_error_messages:
+            raise ContentDecodingError('Got {}'.format(response['message']))
 
     @property
     def fee(self) -> float: