Improve error messages in validation

**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.

No files, tests, or entry points are named. Start by locating the model validation and error-handling code, then identify which validation failures currently lack context; done means the affected errors are more descriptive and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.