Update api documentation

**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 files or tests are named. Start by locating the controller API documentation and the existing controller error-handling entry points, then determine which messages need more detail. Done means the relevant API documentation and error messages clearly describe controller failures and the existing checks still pass.

Written by the indexing model from the issue text.