Update controllers documentation

**Problem**
When working with handlers, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue names controllers documentation and handlers but no file or test. Start by locating the controllers documentation and reviewing its current error-handling guidance. Done means the documentation reflects the requested detailed error-message guidance; clarify whether code changes are also expected before implementing behavior.

Written by the indexing model from the issue text.