Raise NoFeatureFlagFound instead of handle missing flags

The handler is not responsible to handle the feature flag missing. I expect the `KeyError` only if it was not found in any handler.

The current implementation works for only one handler.

This is my first step to create a custom handler. I want to make the core responsible for missing keys.

I'm not sure about the exception name. Django use `DoesNotExist`, SQLAlchemy use `NoResultFound`, but I tried `FeatureFlagNotFound`, `NotFound`, `FeatureNotFound` and others.