Add info about failing discrimintator to UnknownErrorResponse

[ghostbuster91]

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

[Baccata]

In all frankness, if we're gonna target 0.19, I think Gary's code change to remove the type-parametrisation of the discriminator is better to take care of the requirement of "what did we attempt" without needing to amend other functions.

I'd rather you built on top of that, and changed the FailedDecodeAttempt model to something like :

sealed trait FailedDecodeAttempt 
object FailedDecodeAttempt {
  case class UnrecognisedDiscriminator(discriminator : HttpDiscriminator) extends FailedDecodeAttempt 
  case class DecodingFailure(...) extends FailedDecodeAttempt // what's in the current `FailedDecodeAttempt`
}

Therefore, I'm closing this PR.