fix: throw on non-404 stale label removal errors

Addresses CodeRabbit review feedback. Previously, if the API failed to
remove a stale label (e.g. due to rate limits or network issues), the
error was logged but execution continued. Since the new label was
already added earlier in the function, this left the PR in an invalid
state with multiple mutually-exclusive queue labels.

Now, non-404 errors are explicitly re-thrown to fail the workflow and
alert maintainers, preventing broken label states.

Signed-off-by: darshit2308 <darshit2308@gmail.com>

Author: darshit2308

.github/scripts/review-sync/helpers/labels.js

@@ -162,6 +162,7 @@ async function syncLabel(github, owner, repo, pr, dryRun) {
       } else {
         const message = error instanceof Error ? error.message : String(error);
         console.error(`    ✗ Failed to remove "${stale}": ${message}`);
+        throw error; // Re-throw to prevent silently leaving PR in a broken multi-label state
       }
     }
   }