Refactor core for better readability

**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 files, tests, or entry points are named. Start by locating the API error-handling code and reviewing the current error messages and their callers. Done means the relevant errors are more descriptive and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.