512 db redis health

[eastandwestwind]

Purpose

Surface more helpful error messages on app startup for both db and cache connection failures. See associated ticket for examples of the error messages.

Changes

• Removes unneeded dependences from celery in docker-compose.yml. This allows celery to not swallow errors for postgres, specifically.
• Catches exceptions when attempting to init db on app startup
• Attempts to connect to cache on app startup so that connection errors are caught early instead of when trying to process privacy requests
• If connection fails to either db or cache, does not start webserver

Checklist

• Update CHANGELOG.md file
  • Merge in main so the most recent CHANGELOG.md file is being appended to
  • Add description within the Unreleased section in an appropriate category. Add a new category from the list at the top of the file if the needed one isn't already there.
  • Add a link to this PR at the end of the description with the PR number as the text. example: #1
• Applicable documentation updated (guides, quickstart, postman collections, tutorial, fidesdemo, database diagram.
• If docs updated (select one):
  • documentation complete, or draft/outline provided (tag docs-team to complete/review on this branch)
  • documentation issue created (tag docs-team to complete issue separately)
• Good unit test/integration test coverage
• This PR contains a DB migration. If checked, the reviewer should confirm with the author that the down_revision correctly references the previous migration before merging
• The Run Unsafe PR Checks label has been applied, and checks have passed, if this PR touches any external services

Ticket

Fixes #512

[seanpreston]

Thanks @eastandwestwind