GraphQLError double wrapping returning result

I found out something confusing and wanted to know if maybe this could be a bug or if I'm missing something and this is an expected behavior.

This is my `example.py` file. It only has one resolver that raises an error.

```python
from graphql import (GraphQLSchema, GraphQLObjectType, GraphQLField, GraphQLString, graphql_sync, )


def resolve_fail(*args, **kwargs):
    raise ValueError("Some error")


schema = GraphQLSchema(
    query=GraphQLObjectType(
        name='RootQueryType',
        fields={
            'hello': GraphQLField(
                GraphQLString,
                resolve=resolve_fail)
        }))

query = '{ hello }'

result = graphql_sync(schema, query)
```

I'm aware that `result.errors[0]` is a `GraphQLError` exception. But, I was expecting `result.errors[0].original_error` to be `ValueError`. 

However, I can see that `result.errors[0].original_error` is a `GraphQLError` and  `result.errors[0].original_error.original_error` is `ValueError`. 

**Is this ok?**

```bash
>>> print(type(result.errors[0]))
<class 'graphql.error.graphql_error.GraphQLError'>
>>> print(type(result.errors[0].original_error))
<class 'graphql.error.graphql_error.GraphQLError'>
>>> print(type(result.errors[0].original_error.original_error))
<class 'ValueError'>
```

Description