Consider deprecating old handlers 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 handlers API entry points. Review how parser failures are currently reported and determine the intended deprecation path; done should include a decided scope for the API change and more descriptive error messages, with coverage for the affected behavior.

Written by the indexing model from the issue text.