Improve error messages in core

**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.

No files, tests, or parser entry points are named. Start by locating the parser error-handling paths in the repository, then identify the messages that need more context and verify that parser failures are descriptive and useful for debugging.

Written by the indexing model from the issue text.