Add unit tests for parsers

**Problem**
When working with api, 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 API parsers and the existing unit-test conventions. Reproduce the current parser errors, then define tests that demonstrate more descriptive messages; done means the parser tests cover the relevant failures and the error handling provides the requested debugging context.

Written by the indexing model from the issue text.