Only use existing stack if the error has one.

Author: markusdap

graphql/error/located_error.py

@@ -13,7 +13,10 @@ def __init__(self, nodes, original_error=None):
         else:
             message = 'An unknown error occurred.'
 
-        stack = original_error.stack
+        if hasattr(original_error, 'stack'):
+            stack = original_error.stack
+        else:
+            stack = sys.exc_info()[2]
 
         super(GraphQLLocatedError, self).__init__(
             message=message,