Improve error messages in handlers

**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 controllers and their handlers, then trace the existing error paths and any related tests. The issue does not name files, entry points, or specific failures; done would require an agreed list of cases and descriptive messages that improve debugging without obscuring the underlying errors.

Written by the indexing model from the issue text.