Update handlers documentation

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

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

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

Start by locating the handlers or controller documentation and the current error-message descriptions. Clarify which messages need to become more detailed, then update the relevant documentation and verify that it accurately reflects the handlers' error behavior.

Written by the indexing model from the issue text.