Add unit tests for services

**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 or existing tests are named. Start by locating the parser and service implementations and their current test coverage, then reproduce the existing error paths. Done means the relevant parser errors are more descriptive and unit tests cover those cases.

Written by the indexing model from the issue text.