Ensure linting errors trigger a non-zero exit code.

Summary

Fixes error from #9698 where the CLI calling the linting function was not properly reading the return code, so was always returning a falsy value even when there were linting errors, meaning that the exit code was always 0.

References

Root cause of the missed linting here: #12428

Reviewer guidance

Deliberately introduce a linting error. Run yarn run lint-frontend. Ensure that the exit code is 1.

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

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

Asset type	Download link
PEX file	kolibri-0.17.0a0.dev0_git.51.gecde03b2.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.51.gecde03b2-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.51.gecde03b2-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.51.gecde03b2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.51.gecde03b2-0.1.4-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.51.gecde03b2.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.51.gecde03b2-py2.py3-none-any.whl

learningequality / kolibri

Ensure linting errors trigger a non-zero exit code. #12572