Project Management Automation: Call core setFailed with error message

[aduth]

Previously: #20009

This pull request seeks to reintroduce error handling removed as part of #20009. This time, it seeks to report the error using setFailed from the @actions/core package. This is also consistent with other usage a few lines above where a failure of a missing token is reported using this means.

https://help.github.com/en/actions/automating-your-workflow-with-github-actions/creating-a-javascript-action#write-the-action-code

gutenberg/packages/project-management-automation/lib/index.js

Lines 34 to 38 in 2e00995

	const token = getInput( 'github_token' );
	if ( ! token ) {
	setFailed( 'main: Input `github_token` is required' );
	return;
	}

[aduth]

This time it failed, as expected (in other words, "works as expected"):

https://github.com/WordPress/gutenberg/runs/423916394?check_suite_focus=true

I'll now merge #20011 to fix the existing issue with the add-milestone task.