Improve error messages in validation

**Feature request**
It would be great to have export for middleware.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_export` with default `false`.

The issue names no files, tests, or entry points, and its title conflicts with the request about exporting middleware. First inspect the repository's middleware and configuration entry points, then confirm the intended behavior and test location with maintainers. Done means the agreed export option works with its documented default and has coverage.

Written by the indexing model from the issue text.