Add unit tests for services

**Problem**
When working with core, 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 core services and the existing unit-test structure, then inspect how current errors are created and reported. Define the expected detailed messages and add coverage for the relevant service failures; done means the messages improve debugging and the unit tests pass.

Written by the indexing model from the issue text.