Add logger for GraphQL execution exception

[czchen]

The current implementation catch all GraphQL execution exception and return as errors in response. However, the detail information of exception like line number, file name, call stack are missing. It would be difficult to debug when not having these information.

I think it could be better to have a new member logger, and when there is an execution exception, it can use logger.exception to log this exception for debugging.

[syrusakbary]

This issue was fixed in graphql-python/graphql-core#41 (commits graphql-python/graphql-core@f2175f5, graphql-python/graphql-core@0143e08).

Closing issue :)