Improve error message if value is not defined in enum about openapi_first

[ahx]

Hi @johannesluedke! Thanks for the issue.
What do you think about this change? It changes the error to something like:

{
  "errors": [
    {
      "source": {
        "pointer": "/type"
      },
      "title": "value \"unknown-type\" is not defined in enum",
      "detail": "value can be one of pet, plan"
    }
  ]
}