Consider deprecating old lib API

**Problem**
When working with parsers, 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 parser error-handling code and the old lib API referenced by the issue. Review how parser failures are currently reported and identify the relevant tests or entry points; done means the scope is agreed, errors are more descriptive, and any deprecation behavior is documented and covered.

Written by the indexing model from the issue text.