fix: `error.cause` from undici may be instance of Error

[nikhilgupta345]

Resolves #644

Before the change?

I found in my testing that undici may surface the cause property as an Error rather than as a plain string. This meant this if block never got executed and we never bubbled up the actual cause.

After the change?

Correctly handle cause as an Error or as a string, and bubble up the message from the Error, if present.

Pull request checklist

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

• [ ] Yes
• [x] No

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[github-actions[bot]]

:tada: This PR is included in version 8.1.4 :tada:

The release is available on:

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

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