Refactor controllers for better readability

**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.

Start by locating the controllers and the existing error-handling paths for data operations. Review how current errors are produced and define the detailed messages needed; the work is done when those paths provide clearer debugging information and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.