Include `extensions` field in GraphQLError.

The [GraphQL docs](http://graphql.github.io/graphql-js/error/#errors) specify a field called `extensions` is available for a GraphQLError. GraphQL Python does seem to have a ['slot'](https://github.com/graphql-python/graphql-core/blob/master/graphql/error/base.py#L13) for this.

Users could add the field using a wrapper, but it looks like something that could easily be added to the class – unless I'm missing something!

The issue points to graphql/error/base.py, especially the GraphQLError slot around line 13; start there and compare it with the GraphQL documentation for the extensions field. Add support for exposing extensions on GraphQLError, and consider how the documented field shape should be verified.

Written by the indexing model from the issue text.