Add test for role api inputs

Description

This is an attempt to make the api more testable by exposing the earlier added validation errors as proper types instead of plain strings.

Related Issue

#232

Motivation and Context

We should be able to provide proper error messages, when an user input is not as intended and fails validation. This was already in place with the implementation of #232, but only as a string not as typed outputs. Therefore the usability in tests and by application was rather non existend.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.