Improve error messages in api

**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.

Start by locating the API model error-handling entry points; the issue does not name specific files or tests. Determine which model failures currently produce unhelpful messages, then define and verify clearer error output and add coverage for the affected cases.

Written by the indexing model from the issue text.