contextmanager should discard on exception in with block?

[ischneider]

Not sure if you're developing this project any longer but it seems like adding an except block in the open contextmanager would provide the expected behavior on an exception in the outer with block.

Here: https://github.com/abarnert/fatomic/blob/master/fatomic.py#L78

for example:

    except:
        _discard = [True]
    raise