[Box] Fix end signal not enqueued

[parthpuri-elastic]

Closes https://github.com/elastic/connectors/issues/2559

Following changes are included in this PR:

• Ensure end-signals are put to queue even if error is encountered, in all relevant places.

Checklists

Pre-Review Checklist

• [x] this PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check config.yml.example)
• [x] this PR has a meaningful title
• [x] this PR links to all relevant github issues that it fixes or partially addresses
• [x] if there is no GH issue, please create it. Each PR should have a link to an issue
• [x] this PR has a thorough description
• [x] Covered the changes with automated tests
• [x] Tested the changes locally
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)
• [ ] Considered corresponding documentation changes
• [ ] Contributed any configuration settings changes to the configuration reference
• [ ] if you added or changed Rich Configurable Fields for a Native Connector, you made a corresponding PR in Kibana