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.
I really only care to handle _the_ `Error` type, and to wrap other things. If we can handle `Error`s from other domains, that's a plus in my book.

I'll let you both decide :)