Improve error messages in core

**Problem**
When working with data, 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 locating the core error-handling paths involved when working with data. Before changing code, clarify which errors need improvement and what details each message should include. Done requires agreed error cases and consistently more descriptive messages, with verification defined for those cases.

Written by the indexing model from the issue text.