Refactor config 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.

No file, test, or entry point is named. Start by tracing configuration and model-related error handling in the terminal agent, then identify the existing messages and tests before defining the desired wording. Done means the scope is agreed and error messages are more descriptive without changing unrelated behavior.

Written by the indexing model from the issue text.