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.

The issue names controller handlers but no files, tests, or entry points. Locate the controller error-handling paths and review their current messages first; clarify which errors need coverage before making changes. Done means the affected errors provide detailed, useful messages and the relevant behavior is verified.

Written by the indexing model from the issue text.