client: Better error handling in Watcher.Close()

[Jille]

The previous implementation might've ignored a real error if the last error happened to be context.Cancelled.

While we're at it, might as well use the new errors.Join() to actually return all errors.

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[k8s-ci-robot]

Hi @Jille. Thanks for your PR.

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[Jille]

Hmm, I just realized errors.Join() will always wrap the error. Do we care that the return value might have a layer of wrapping?