Add unit tests for core

**Problem**
When working with api, 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 path, or entry point is named. Start by locating the core API error-handling paths and existing unit tests, then identify which messages are currently insufficient. Done means adding unit coverage for the relevant error cases and making the resulting messages more descriptive.

Written by the indexing model from the issue text.