Surface checks that failed to run

[sanketio]

A failed check request is no longer swallowed: the completion notice names the checks that did not run, instead of reporting "No errors found".

• /wp-admin/tools.php?page=plugin-check — Performance category on an active plugin: notice lists the five runtime checks whose requests return 400
• Same screen, Plugin Repo category: unchanged, reports the error/warning counts
• npm run lint-js

Part of #1445.

[github-actions]

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