Fix feature manager

[danixeee]

Description (e.g. "Related to ...", etc.)

Fix adding help attributes to registered feature/command handlers.

This fix makes sure that this piece of code is properly executed and handler's return values are type-checked.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)