fix: fail on real label removal errors

[thc1006]

Description

removeLabels() caught every error from octokit.issues.removeLabel and only logged it with core.debug, so /remove, /hold cancel, and /lgtm cancel reported success even when the removal failed with a 403, a 500, or a network error, leaving the label in place.

This keeps the benign case quiet and surfaces the rest: a 404 means the label is already gone, so it stays a debug log, but any other error is rethrown. The callers cancelLabel and remove already propagate, so a real failure now reaches core.setFailed.

Fixes #105

Testing

• npm run all passes (build, lint, pack, test)
• a failing removal (500) fails the Action through core.setFailed, via /remove and via the /hold cancel path through cancelLabel
• a label that is already gone (404) is ignored and the Action still succeeds
• confirmed the failure test fails without the change

Checklist:

• I ran npm run all to lint and build my code
• No new dependencies
• I have performed a self review of my own code
• I have commented my code, particularly in hard to understand areas
• I have made corresponding changes to the documentation (no documentation changes needed)
• My changes generate no new warnings