Consider deprecating old controllers API

**Problem**
When working with data, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue does not name files, tests, entry points, or a specific controller API. First clarify which old controllers API is in scope and which error cases need more detail; then identify the affected error-handling paths and define the expected messages and deprecation outcome before implementation.

Written by the indexing model from the issue text.