Expose details for returned errors

[akupila]

Exposes the error as graphql.Error and returns error details in case they are present in the error response.

The user should assert the error type:

err := cli.Run(...)
if errs, ok := err.(graphql.Errors); ok {
  // Range over errs
}

// or with go 1.13
var errs graphql.Errors
if errors.As(err, errs) {
  // Range over errs
}

Included:

• message: As before
• locations: Locations in the query that caused errors
• paths: Response paths that returned errors
• extensions: Optional extensions set by the server. The spec specified it's a map but nothing more.

The format matches the June 2018 spec for errors: https://graphql.github.io/graphql-spec/June2018/#sec-Errors

This PR is very similar to some previous ones:

• https://github.com/machinebox/graphql/pull/24, https://github.com/machinebox/graphql/pull/34 - Slightly different way of accessing extensions (map[string]interface{} instead of method) + includes locations and path
• https://github.com/machinebox/graphql/pull/37 - This PR also prints multiple returned errors (slightly different format) and also allows inspecting the errors for additional values (locations, path, extensions)

[bcaw]

Could we get some love on this to get merged? It would be great to have better visibility into errors using this lib.

[peterhellberg]

Is there anything blocking this PR from getting merged? (or #24)