Project Management Automation: Check error object for parsed errors

[aduth]

Previously: #20011

This pull request seeks to fix the check to determine whether an error thrown in the creation of the milestone during the add-milestone automation task is an error representing a duplicate entity.

Based on related resources, it should be the case that this can be determined using the parsed errors property of the error object.

• https://github.com/octokit/rest.js/issues/684
• https://developer.github.com/v3/#client-errors
• https://github.com/octokit/request.js/blob/2a1d768/src/fetch-wrapper.ts#L79-L80

[aduth]

The failing build does not seem that it should be related, and is of the same sort of intermittent failure that #18773 had originally hoped to address (ZIP failure download). Since it's not related, and I don't think the E2E jobs are particularly relevant for this package, I'm going to merge anyways.

[aduth]

The action ran successfully this time 🎉

https://github.com/WordPress/gutenberg/runs/424037572?check_suite_focus=true

add-milestone: Fetching current milestone
add-milestone: Fetching `package.json` contents
add-milestone: Current plugin version is 7.4
add-milestone: Creating 'Gutenberg 7.5' milestone, due on 2020-01-27T00:00:00.000Z
add-milestone: Milestone already exists, proceeding with assignment
add-milestone: Fetching all milestones
add-milestone: Adding issue #20014 to milestone #114
main: All done!

You can also see in the history of this pull request that the bot assigned the milestone:

#20014 (comment)

[epiqueras]

Nice work!

[aduth]

Thanks for the timely reviews!

The continued iterations would seem to highlight that this is worth some consideration about how to improve the workflow to changes (verifying correctness) in these automations.

[epiqueras]

I think actions have a testing framework now.