Queryables 404 for non-existent collection ids

[mmcfarland]

Related Issue(s):

• pgstac backend returns valid queryables for non-existing collection ids #481
• Don't return valid queryable object for invalid collection  pgstac#145

Description:
Return a not-found response if pgstac does not return a queryable object when passed a non-existing collection id.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.