Make Celery backend optional

[oh2fih]

In case the celery module requirements are missing, handle the exceptions caused by the missing dependencies, and continue without the Celery backend support.

At least a partial solution for the issue #295.

- CveXplore.main - INFO     - Using mongodb as datasource, connection details: None
- CveXplore.main - INFO     - Failed to load Celery TaskHandler; continuing without the optional Celery backend.
- CveXplore.main - INFO     - Initialized CveXplore version: 0.3.33

[oh2fih]

@P-T-I I just tested this together with cve-search/cve-search#1108 (and #296 is just a documentation enhancement).

• cvesearch.db_repopulate.target i.e. cve-search/sbin/db_updater.py -v -f
• cvesearch.db_updater.service i.e. cve-search/sbin/db_updater.py
• cvesearch.web.service
  • GUI (frontpage, CVE pages)
  • /api/last
  • /api/cvefor/

The API is still timing out from time to time and I only get < 2 GB to the database instead of 3.5 GB, but again it's not related to any of these changes anyway. (Later tonight I'll probably get a confirmation from another instance that re-population on CVE-Search 5.0.3 with CveXplore 0.3.30 partially fails the same way.)