The descriptions provided whenever a REST API check failed lacked details that would help in troubleshooting any issues, most notably the actual REST API endpoint that was being tested.
Adding this vital piece of detail, along with a revamp of the error messages shown, where previously there was a risk that the markup of your website was included as an error message, improves the overall user experience, and gives more accurate details that can be looked when looking up how to fix or improve the behavior of the REST API.
Props dingo_d, shetheliving.
Fixes#54617.
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