Added basic readiness check for registry API

[soyarsauce]

What this PR does

Fixes #2992

Checklist

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

[t83714]

I think just some minor error handling changes are required.

[t83714]

Tested in the cluster and it seems most exceptions are not captured.
e.g.

java.util.NoSuchElementException: key not found: to_regclass
OR org.postgresql.util.PSQLException: Connection to registry-db:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.
OR org.postgresql.util.PSQLException: FATAL: role "client" does not exist

I think you probably need to capture any failure from the DB.readonly method as the execution is happening there.
you might want to try scala's util.Try instead of try/catch --- it's more common in scala as it's more flexible / offer more streamlined exception processing
https://www.scala-lang.org/api/2.9.3/scala/util/Try.html

You can do something like this:

import scala.util.{Try, Failure, Success}
Try{
        DB readOnly { session =>
          recordPersistence.getReadiness(Some(logger))(session)
        }
    } match {
        case Success(Some(true)) => complete("OK")
        case Success(Some(false)) | Success(None) =>
          complete(StatusCodes.InternalServerError, "Database not ready")
        case Failure(exception) => complete(StatusCodes.InternalServerError, "Database not ready Other")
    }

The recordPersistence.getReadiness doesn't seem to need to capture any exceptions (so you don't have to capture at multiple place) nor has a chance to produce a NONE. I guess a Boolean could simpler

[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

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