Tweaks to CLI and message extraction utility function

[rtibbles]

Summary

• Properly updates our programmatic invocation of help inside CLI commands to prevent trying to redefine an existing command (needed since the upgrade to commander v12).
• Ensures the message extraction utility function always returns an object, just empty in the case where a file could not be parsed.

References

Fixes issues seen by @marcellamaki while trying to do a string audit.

Reviewer guidance

Run yarn exec kolibri-tools <name of command> for several of the i18n commands in cli.js to confirm it properly shows the help messages.
Run yarn exec kolibri-tools i18n-audit -- --output-file audit.csv --ditto-file <path to ditto export csv> --pluginFile ./build_tools/build_plugins.txt with a file in one of the plugin src directories that has invalid syntax in it, and confirm it does not error.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.42.g7089b8fe-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.42.g7089b8fe-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.42.g7089b8fe-0.4.2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.42.g7089b8fe-0.1.3-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.42.g7089b8fe.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.42.g7089b8fe-py2.py3-none-any.whl