Use `toString` check rather than `instanceof` check.

Re: discussion in #1 Should fix wonky js context things.

This seems to provide the semantics of what the type specifies, since `throw`ing if we didn't immediately get an `Error` actually introduces back an `Exception` effect. which `catchException` is supposed to remove.
Oh, and if you have a different or better way to do this, please, relay the info or submit a pr or whatever. 

As I said, I don't really care what happens here, so long as it's not a runtime error I can't do anything about.