Add unit tests for lib

**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 implementation and related code under lib, then inspect how current errors are produced and whether existing tests cover those paths. Define the expected descriptive messages and add unit coverage for parser failures; the work is done when the tests document the improved errors and pass.

Written by the indexing model from the issue text.