Consider deprecating old lib API

**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 controller error-handling paths and references to the old lib API. Clarify which API should be deprecated and which error messages need detail; done means the deprecation behavior and controller errors are specified and covered by the relevant checks.

Written by the indexing model from the issue text.