Make struct mode access errors consistent with Python

[omry]

When struct mode is enabled, DictConfig behaves like a normal dictionary and also like a real object.

Make sure exceptions thrown are consistent with Python:

1. foo["not_found"] should raise key error
2. foo.not_found should raise attribute error.

Note that while not in struct node, both of those return None instead of raising an error.

[omry]

done