Add unit tests for controllers

**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.

No file or test path is named; first locate the controller implementations and any existing unit-test entry points, then reproduce the current error messages. Done means controller error cases have focused unit coverage and the resulting messages are more descriptive, with the test suite passing.

Written by the indexing model from the issue text.