Guide to returning your own errors #101

The docs explain the built-in not-found and validation errors but not how to add your own, such as a 409 when a review already exists. Engineers will hit that in their first week.

Phase 1: The guide

• A section or page per template: add an Application exception, map it in the error handler, filter or function result, document it, and test it, following errors-and-exceptions

Checks: the site builds, and the example compiles in a freshly generated solution.