Fix/mip 632/celery app fixes

Created by: ThanKarab

Changelog:

• Celery Wrapper (app) refactoring and bugfixing. Jira
  • Celery Wrapper handled various exceptions and reinitialized the internal celery app when needed.
  • Integration tests added for celery wrapper specifically.
• Added separate handling for CeleryConnectionError and CeleryTaskTimeoutException in the api error handlers.
• Other minor improvements and bug fixes.