(graphcache) - Fix uncached GraphQLError fields making it impossible to get first API result

[kitten]

This is a follow-up for #1356

Summary

The #1356 change introduced behaviour that prevents null values from fields that have errored and are associated with GraphQLError from being cached. This caused a regression where when we "read-back" results from the cache that have just arrived from the API these fields would cause a cache miss:

https://github.com/FormidableLabs/urql/blob/6a687091b67dc3d0ee78353ca79b0aa401ba983c/exchanges/graphcache/src/cacheExchange.ts#L237-L243

We can prevent this by also tracking errored fields in the query and ignoring these uncached fields there too by replacing them with null while we still have access to the GraphQLErrors.
This is preferable to other solutions as we've gotten rid of comparing the cached result to the API result in #1196 as we had trouble differentiating between null selection sets from previous selections and null values from the API result.

No changeset is needed since the PR is unreleased.

Set of changes

• Add (previously failing) test for retrieving a result with null'd fields due to GraphQLErrors from an API
• Track ctx.__internal.path in all environments in query too
• Check for errors before aborting a selection in query and set the field to null if it does have an error

[changeset-bot]

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Latest commit: 1a253a5

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[wgolledge]

This makes sense to me! The result.error was probably going to have to be brought in at some point anyway