text: Title: Accessing unselected columns should raise an error rather that return None
  Body: Hello!
  
  When one select columns with `Table.select()`, the returned object will have only part of attributes. Currently, gino (0.4.1) would return `None` when accessing attributes that weren't selected. However, accessing unselected attributes usually mean that there is a bug somewhere so it would be better to throw an appropriate error.