BoxMapping exists()

[pasraj]

If we call app.state.box_name[box_key].exists()

It should return 0 or 1, but when box_key is wrong, it raises an error.

Expect: Return a boolean value o or 1

Got : invalid Box reference

https://github.com/algorand-devrel/beaker/blob/master/beaker/lib/storage/box_mapping.py