btcd: fix nonzero exit code on sigint when indexing cfindex

[allocz]

Change Description

Fixes #2530 by returning nil instead of error when newServer returns error because an interrupt was received.

Steps to Test

1. Sync btcd up to some height, then stop the node;
2. Start btcd with --dropcfindex, btcd will stop after dropping the index;
3. Start btcd again and send ctrl+c after seeing a log containing [INF] INDX: Catching up indexes from height;
4. The status code should be zero with the patch, and not zero without the patch.

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

📝 Please see our Contribution Guidelines for further guidance.