> the Error type, the superclass of exception

> in Either<Error, Foo>, Error is not that

This is confusing. On the JVM `Error` and `Exception` are both subclasses of `Throwable`. If you meant a specific error type, give it another name. (I used `MyError` in the comments above.)

> sealed class ChangePasswordErrors {

If some kind of failure has to be handled, In Kotlin you can write:

    sealed class ChangePasswordResponse {
        class Success(): ChangePasswordResponse()
        class NotAuthenticated() : ChangePasswordResponse()
        ...

> Just call map, flatMap, fold.

Enjoy your callback hell.