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 file, test, or entry point is identified. Start by locating the validation and error-handling paths in the Shell-based terminal agent, then review existing messages and tests before defining which details should be included and how to verify the improved output.

Written by the indexing model from the issue text.