When `sourceStream` errors, yield a `{ errors: [...] }` response

At a glance

• Identifier: #1127
• Stage: RFC0: Strawman
• Champion: @benjie
• PR: When `sourceStream` errors, yield a `{ errors: [...] }` response

Timeline

• Added to 2025-01-09 WG agenda
• Commit pushed: Merge branch 'main' into streams-editorial-suggestions on 2024-12-05 by @benjie
• Added to 2024-12-05 WG agenda
• Mentioned in 2024-12 WG notes
• Spec PR created on 2024-11-21 by benjie
• 3 commits pushed on 2024-11-21:
  • Don't complete with error. by @benjie
  • Don't use a new algorithm, no need to DRY by @benjie
  • Internal errors should be passed up the chain by @benjie
• Commit pushed: feedback and use definition syntax on 2024-06-11 by @leebyron
• Commit pushed: Editorial changes for Event Streams on 2024-06-06 by @leebyron

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See https://github.com/graphql/graphql-spec/pull/1099/files#r1799509253

Currently if sourceStream generates an error, then responseStream repeats the error. This is the behavior implemented in graphql-js and is problematic.

GraphQL captures execution errors and wraps them in an { errors: [...] } payload for query and mutation operations; it should do the same for stream errors in a subscription operation.

This PR makes this change. It is technically a breaking change, I think, so will require​ careful thought.