Return non-zero exit status for invalid commands

[neoaggelos]

Summary

Closes #4091

Changes

• Add a common NeedSubcommandRunE function, printing help message and returning an error
• Set RunE for all top-level commands of ttn-lw-cli and ttn-lw-stack to return an error.

Testing

Test locally

Regressions

None

Notes for Reviewers

Draft since this is not a too elegant solution, see #4091 (comment)

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• 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.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.