Refactor core for better readability

**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; start by locating the model-related error handling in the core. Done means the relevant errors are more descriptive and the handling is easier to read, but the issue provides no concrete acceptance criteria or validation command.

Written by the indexing model from the issue text.