Future<Void> BoundWithAPI.isValid, not Future<Boolean>

The BoundWithAPI.isValid method returns a Future<Boolean>. If the validation fails the Future succeeds with a false boolean.

However, Futures are designed to return failures as a failed Future.

Therefore Future<Void> should be returned so that the caller of isValid can better handle the error.

The org.folio.rest.exceptions.ValidationException can be used. This also fixes the error response that currently doesn't comply with the RAML error response spec:

{
  "map" : {
    "holdings" : {
      "list" : [ "Holdings record 5dad1351-1a77-447b-b5fe-a14c2ed280e2 not found." ],
      "empty" : false
    }
  },
  "empty" : false
}