apollo-errors: Spec compliant error

Recently, GraphQL Response Specification got updated with a suggestion to place custom data in error in the extensions object. The spec can be found here https://github.com/facebook/graphql/blob/master/spec/Section 7 -- Response.md

An excerpt of the relevant section:

GraphQL services may provide an additional entry to errors with key extensions. This entry, if set, must have a map as its value. This entry is reserved for implementors to add additional information to errors however they see fit, and there are no additional restrictions on its contents.

{
  "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"
      }
    }
  ]
}