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':
+        temp_error_messages = [
-            raise ContentDecodingError('Unable to decode bittrex response')
+            '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:

freqtrade/tests/test_exchange_bittrex.py

@@ -0,0 +1,32 @@
+# pragma pylint: disable=missing-docstring,C0103
+
+import pytest
+from requests.exceptions import ContentDecodingError
+
+from freqtrade.exchange import Bittrex
+
+
+def test_validate_response_success():
+    response = {
+        'message': '',
+        'result': [],
+    }
+    Bittrex._validate_response(response)
+
+
+def test_validate_response_no_api_response():
+    response = {
+        'message': 'NO_API_RESPONSE',
+        'result': None,
+    }
+    with pytest.raises(ContentDecodingError, match=r'.*NO_API_RESPONSE.*'):
+        Bittrex._validate_response(response)
+
+
+def test_validate_response_min_trade_requirement_not_met():
+    response = {
+        'message': 'MIN_TRADE_REQUIREMENT_NOT_MET',
+        'result': None,
+    }
+    with pytest.raises(ContentDecodingError, match=r'.*MIN_TRADE_REQUIREMENT_NOT_MET.*'):
+        Bittrex._validate_response(response)