Add unit tests for models

**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 models referenced in the issue, then inspect the existing unit-test structure and current error-handling behavior. The work is done when model tests cover the relevant cases and controller errors provide the requested descriptive messages, but the issue does not identify specific files or cases.

Written by the indexing model from the issue text.