Review handling of consumed sandboxes in the JS API #108

I am not a big fan of getters throwing errors, as that's an unexpected behaviour for a getter.

At the same time, I am not sure what would be the correct result to return form a consumed sandbox, maybe false? maybe null? but I guess returning null is not too different from throwing.

I propose we leave it as it is right now, and we get someone with more JS API design experience to take a look into it. I'll open an issue so that we don't forget about it.

Originally posted by @jprendes in #69 (comment)