BoxMapping exists() #243
Description
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