formatError: put all extensions inside 'extensions' property

[IvanGoncharov]

Mirrors graphql/graphql-spec#407

This is breaking change, however, extensions is the relatively new feature that was added in 0.12.0.
Also, it could be easily workaround by providing formatError property to a server:
https://github.com/graphql/express-graphql#options
https://github.com/apollographql/apollo-server#options

Unintended changes: I didn't want to add extensions: undefined to every error inside tests so I refactored them.

[leebyron]

This is great - just some small suggestions inline.

We'll also need some kind of adoption strategy to help users with the breaking change

[leebyron]

The extensions key should only exist if there are extensions, so perhaps this should if (error.extensions) and return different shapes

[leebyron]

Based on spec description, this should be:

+extensions?: { [key: string]: mixed }

[IvanGoncharov]

@leebyron Thanks for review 🎉

This is great - just some small suggestions inline.

Fixed in 80ca852. I tried dozens of ways how to initialize optional non-undefined readonly property and it's the simplest one I found.

We'll also need some kind of adoption strategy to help users with the breaking change

JSON.stringify(error) is not affected.
As for formatError, if we assume people read Release notes before updating to the new version than it's easy. You just need to write that in GraphQL server of your choice express-graphql, apollo-server, etc. you need to supply formatError as:

import { formatError as baseFormatError, /* ... */ } from 'graphql';

{
  // other options
  formatError(error) {
    const { extensions, ...rest } = baseFormatError(error);
    return { ...extensions, ...rest };
  },
}

[leebyron]

Nice - Let's get #1305 landed first and I'll let you rebase over that, otherwise I really like this change.

[IvanGoncharov]

@leebyron I rebased this PR on top of master.

[leebyron]

Great work on this!