Added basic readiness check for registry API

[soyarsauce]

What this PR does

Fixes https://github.com/magda-io/magda/issues/2992

Checklist

• [x] There are unit tests to verify my changes are correct or unit tests aren't applicable
• [x] I've updated CHANGES.md with what I changed.
• [ ] I've linked this PR to an issue in ZenHub (core dev team only)

[soyarsauce]

I'm sorry @t83714 I've run out of time for this one, are you able to do the error handling changes you have in mind, then get this in? 🙏 I think it would still be extremely useful for the registry API to have this check!

[t83714]

@soyarsauce Thanks for your work on this and just leave to me. I will get this done after this one #3024

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}