Improve ResponseCodeError API

[AnthonyMDev]

This error is really difficult to parse information from. See #2419 for more info.

[dfperry5]

@AnthonyMDev Any chance this is still an open issue? :) I wouldn't mind taking it on. This would be my first time contributing to Apollo iOS though.

[AnthonyMDev]

I'd be happy to have you work on this @dfperry5! Thanks for offering to contribute.

It actually looks like the data does contain a spec compliant GraphQLError response. So we should probably be exposing that data as a GraphQLError struct.

The right way to do this would involve separating the JSON serialization interceptor from the parsing of the actual GraphQL result, but that is going to require a lot of restructuring of our network APIs that we are going to do in the 2.0 release.

But in order to improve this for now without breaking things, I think the right approach would be for us to:

• Add a new optional computed property graphQLError: GraphQLError? to ResponseCodeError.
  • This computer property would deserialize the JSON itself and if it is able to parse then parse the error from that into a GraphQLError struct if it exists.

[dfperry5]

Awesome! @AnthonyMDev - I will give it a try!

[dfperry5]

@AnthonyMDev - could you take a peak at this? https://github.com/apollographql/apollo-ios/compare/main...dfperry5:apollo-ios:Issue-2426-Improve-ResponseCodeError-API?expand=1

It's my first pass, I'm not overly thrilled with it :P Not sure if there is something more obvious I am missing.

[AnthonyMDev]

This looks like what we want right now. If you can just add another test that an error that doesn't have a GraphQLError returns nil from error.graphQLError and then add a line of documentation to the graphQLError property this should be ready for a PR!

Thanks so much for doing this!

[dfperry5]

#3123 :) Thanks for guiding me! Let me know if there are any chances.