Add unit tests for parsers

**Problem**
When working with core, 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 core parser implementation and its existing unit-test structure. Identify the parser errors that currently lack detail, then add tests covering the improved messages; done means the relevant parser tests pass and failures provide more descriptive debugging information.

Written by the indexing model from the issue text.