[3.5] fix(defrag): handle errors during defrag

[ghouscht]

Backport of #18822 to 3.5.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

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[ghouscht]

@ahrtr PR for the 3.5 backport. Should I squash the commits into one?

[ahrtr]

/ok-to-test

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ghouscht, serathius

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[ivanvc]

Link to #18810