Convert REST response type to tagged union

[ecxyzzy]

Currently, to determine whether a RawResponse<T> is a Response<T> or an ErrorResponse, we test whether the payload field exists. This is kind of cumbersome and is why we ship an isErrorResponse function with the types package, but we could just add success: true to the former variant of RawResponse<T> and success: false to the latter.

This would make error checking as simple as testing res.success, and would also allow consumers of the types package to only add it as a devDependency (not sure if the latter matters as much though).