fix(respecDocWriter): show actual error instead of 'Error: undefined'

[marcoscaceres]

The timeout rejection in evaluateHTML rejects with a plain string, not an Error object. When caught, err.stack is undefined, producing 'Error: undefined'. Now falls back to err itself when stack is unavailable.

Closes #5080

[Copilot]

Pull request overview

Improves the fatal error message emitted by respecDocWriter during static HTML generation so that non-Error rejections (e.g., plain strings) show meaningful details instead of Error: undefined.

Changes:

• Update generateHTML() error formatting to fall back to the caught value when err.stack is unavailable.

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[marcoscaceres]

evaluateHTML rejects with a plain string

@copilot, would it be possible/better to reject evaluateHTML with an actual error somehow? Then we would always have a stack.

[Copilot]

evaluateHTML rejects with a plain string

@copilot, would it be possible/better to reject evaluateHTML with an actual error somehow? Then we would always have a stack.

Fixed the root cause in b27310e — setTimeout(() => reject(msg), ms) now becomes setTimeout(() => reject(new Error(msg)), ms), so evaluateHTML always rejects with a proper Error object and we always have a stack. The err.stack || err workaround in generateHTML is reverted since it's no longer needed.