Fix handling of application packages when not found

[kschiffer]

Summary

OS Counterpart of https://github.com/TheThingsIndustries/lorawan-stack/pull/3380

Changes

• Ensure that 404s will not cause the request handler to throw, since it is an expected result if a package association does not exist

Notes for Reviewers

See https://github.com/TheThingsIndustries/lorawan-stack/pull/3380

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• [x] Documentation: Relevant documentation is added or updated.
• [x] Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• [x] Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.