Improve error messages in core

**Problem**
When working with core, 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 core error-handling paths in the repository and reproduce the current errors. The issue does not name files, entry points, or tests, so first establish which errors are in scope and how their messages should become more descriptive. Done means the agreed error cases provide useful debugging context and are covered by appropriate checks.

Written by the indexing model from the issue text.