Update validation 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 no file, test, or handler entry point. Start by locating the validation documentation and the handler error paths, then compare the current messages with the requested descriptive behavior. Done means the relevant documentation and error messages clearly explain validation failures.

Written by the indexing model from the issue text.