raft: match exact storage errors in bootstrap test

[NasitSony]

Summary

Replace generic error assertions in the bootstrap test with exact error matching.

Motivation

The test previously asserted only that an error occurred. Matching the exact error documents the expected behavior more clearly and provides stronger regression protection.

Testing

Ran:

go test ./...

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @NasitSony. Thanks for your PR.

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