Do not ignore database connection errors

What does this MR do?

Whenever sidekiq loses a database connection the jobs will just immediately return success without doing anything because we return fake settings.

This MR changes the way database connection errors are handled for ApplicationSetting. Now instead of returning Gitlab::FakeApplicationSettings whenever we lost the database connection we limit that behaviour to rake tasks. Otherwise we just raise an exception and sidekiq jobs will be rescheduled instead of silently dropping.

#205640 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team