Pass through errors encountered when running final execute

[abernix]

This is a tracking issue to account for the need to re-implement a different approach for achieving subgraph error pass-through behavior, following up on the first attempt to do so in #159 (comment).

While that was an improvement in principle, it:

• has a bug that was first captured in Gateway: pass through errors encountered when running final execute #159 (comment) To quote that here:

  I think there's a bug here. Let's say we have a query { x } where x is a non-nullable field, and the federated execution has x throw an error. This turns all of data into null (not { x: null }). But then this re-execution adds another error saying that the non-null field is null.

  Take a look at https://codesandbox.io/s/angry-raman-uuzuf?file=/src/index.js for an example of what's going on here.

• Sometimes — unexpectedly to current users, at least — breaks client expectations.

As of now, the pain points seem to be outweighing the gains. I won't dispute the original PR having definitely solved a problem, but the original PR didn't have an issue that it closed nor did it introduce tests so I'm not super crisp on the extent. Given that we've now a non-zero amount of feedback that the approach is causing problems, we'll need to revert this and revisit this when time allows with a slightly different approach.

Ref: #159
Ref: apollographql/apollo-server#5550
Ref: #974
Ref: apollographql/apollo-server#4523

[abernix]

It has been reverted with #982 and will soon be cut into a release of @apollo/gateway@0.38.1, which is tracked in ##983.

[abernix]

Gathering use-cases: Some examples of errors that may come from subgraphs that would be beneficial are things like:

• https://github.com/graphql/graphql-js/blob/fbe8402db3b7751c5b1340886c3b09a7f36aee58/src/execution/execute.js#L401-L404
• https://github.com/graphql/graphql-js/blob/fbe8402db3b7751c5b1340886c3b09a7f36aee58/src/execution/execute.js#L237-L239
• cases where a subgraph returning illegal values, such as Ints that are out of the range of valid GraphQL values.