errors: adds OriginalError support

[chris-ramon]

Overview

• Build on top of: Added Custom Error Formatter #379 — Thanks a lot @racerxdl! 👍

• This PR adds support for retrieving the original error via FormattedError.OriginalError().

  • No breaking changes.

  • Does not expose original errors when JSON serializing since it is available only by a method and the internal field originalError is unexported.

• Matches graphql-js reference implementation originalError (naming and private variable):

  declare class GraphQLError extends Error {
    constructor(
      message: string,
      nodes?: $ReadOnlyArray<ASTNode> | ASTNode | void | null,
      source?: ?Source,
      positions?: ?$ReadOnlyArray<number>,
      path?: ?$ReadOnlyArray<string | number>,
      originalError?: ?Error, <---------------------------------------------------------
      extensions?: ?{ [key: string]: mixed },
    ): void;

  Ref: Link

Test plan

• Unit tests.
• Adds tests for handling errors from resolver.
  • Returning an errors.New(...) error.
  • Returning a custom error.
  • Returning a custom error with extensions.
  • Panic from resolver.

[limoli]

When are you going to merge this? :) @chris-ramon

[coveralls]

Coverage remained the same at 91.87% when pulling 7995f6a on formatted-error-private-original-error into 8000299 on master.