BUG-FIX: add check in _get_component to check if component is in the model

Fixes confusing expection when m.fit_component is called with a
component which is not in the model.
Fix wrong indention in m._get_component
Add test: checking if component is in model when using m.fit_component
Merge pull request #394 from magnunor/ENH_check_if_component_is_in_model

Enh check if component is in model