Improve error messages in core

**Problem**
When working with models, 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 names no files, tests, or entry points. First inspect the project structure to locate core model error handling and review existing error messages. Done requires an agreed scope for more descriptive errors plus coverage demonstrating the improved messages.

Written by the indexing model from the issue text.