feat: include mutation error items in error message

[rexxars]

When you encounter mutation errors, the error message/staack from the client can be less than helpful:

ClientError: Mutation(s) failed with 1 error(s)
    at onResponse (/my/app/node_modules/@sanity/client/dist/index.cjs:78:13)
    at applyMiddleware (/my/app/node_modules/get-it/dist/index.cjs:60:15)
    ...

The response body does usually contain more information, but holds it in an items array. It is not obvious that this is available - you can technically get it at error.details, but it's typed as any and not documented. Ideally we'd introduce a new MutationError class with better typing, but that would be a breaking change (people may be relying on instanceof ClientError checks or similar), so will leave that for a later PR.

This PR provides a slight improvement on the situation, in a way that should be backwards compatible:

• It extracts the description property of the first 5 error items and includes them in the error message
• Includes the number of additional errors there were

So the resulting error might look like the following:

ClientError: Mutation(s) failed with 6 error(s):
- Malformed document ID: "#some_invalid-id!"
- Malformed document ID: "@ruby_bird@"
- Malformed document ID: "!cant_contain_that"
- Malformed document ID: "what$about!this?"
- Malformed document ID: "%so_many_percent%"
...and 1 more
    at onResponse (/my/app/node_modules/@sanity/client/dist/index.cjs:78:13)
    at applyMiddleware (/my/app/node_modules/get-it/dist/index.cjs:60:15)
    ...

[stipsan]

Nice! 👍

[bjoerge]

This is great! 💯

Just a minor nit, otherwise LGTM

[github-actions]

🎉 This PR is included in version 5.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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