Refactor unreachable code

[Stavrospanakakis]

return nil, NotFoundError is unreachable code so I refactored the method.

func (store *storage) PlaygroundGet(id string) (*types.Playground, error) {
	store.rw.Lock()
	defer store.rw.Unlock()
	if playground, found := store.db.Playgrounds[id]; !found {
		return nil, NotFoundError
	} else {
		return playground, nil
	}
        // unreachable code
	return nil, NotFoundError
}