Errors: support extensions

[ghostdogpr]

Add support for the extensions data for each error as defined in the GraphQL spec:

{
  "errors": [
    {
      "message": "Name for character with ID 1002 could not be fetched.",
      "locations": [ { "line": 6, "column": 7 } ],
      "path": [ "hero", "heroFriends", 1, "name" ],
      "extensions": {
        "code": "CAN_NOT_FETCH_BY_ID",
        "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
      }
    }
  ]
}

See initial discussion in #140

[ghostdogpr]

We discussed returning innerThrowable and possibly explanatoryText as extensions.