Should we allow ValidationException in the REST Catalog Schema?

[Alex Stephen via dev]

Hello,

Certain validation errors (example: sort orders referencing an invalid field
<https://github.com/apache/iceberg-python/pull/2937>) will cause the
iceberg-rest-fixture to throw the following 400 error.

 "error": {
     "message": "Cannot find source column for sort field: identity(9999)
ASC NULLS FIRST",
      "type": "ValidationException",
      "code": 400,
  }

ValidationException isn't currently a recognized exception type in the
spec. What should the expected behavior be here?

I'd personally expect that we would return:

"error": {
     "message": "Malformed request: Cannot find source column for sort
field: identity(9999) ASC NULLS FIRST",
      "type": "*BadRequestException*",
      "code": 400,
  }

This would conform with how we handle HTTP error code 400.

Please let me know your thoughts.

-- Alex Stephen