Add unit tests for controllers

**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.

Locate the controllers or handlers mentioned in the issue and inspect existing unit-test patterns first. Identify the current error paths and agree on which descriptive messages and controller behaviors should be covered; done means the tests pass and the intended error details are verified.

Written by the indexing model from the issue text.