mixins: raise ServerError with kwargs only

Author: zokalo

src/httpapiclient/mixins.py

@@ -41,7 +41,7 @@ def clean_response(self, response, request):
         try:
             Draft4Validator(schema).validate(result)
         except ValidationError as e:
-            raise self.ServerError(e, schema=schema, level='json')
+            raise self.ServerError(description=str(e), schema=schema, level='json')
 
         return result