Backport: Keep ShaAttribute from halting startup when we can’t connect to a database

What does this MR do?

This is a backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5502

Catch the exception in ShaAttribute when it tries to connect to database that doesn't exist.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Brings the ShaAttribute implementation back inline with the EE version, and handles exceptions from the database better.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [-] Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5794