Use Result type for returning success result or error

Where we currently use `void onDone​(@Nullable java.lang.Throwable e, @Nullable T result)`.

Similar to [Result](https://developer.apple.com/documentation/swift/result) type we use in our Swift SDK. Or similar to [kotlin.Result](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-result/).

In Java it may be sth like this:

```java
public class Result {
private T result;
private Throwable error;

private Result(T result, Throwable error) {
this.result = result;
this.error = error;
}

public static Result success(T result) {
return new Result<>(result, null);
}

public static Result error(Throwable error) {
return new Result<>(null, error);
}

public T getResult() {
return result;
}

public Throwable getError() {
return error;
}

public boolean isSuccess() {
return error == null;
}

public boolean isError() {
return error != null;
}
}
```

For now opened to collect opinions, unfortunately this will break API.

Start by locating the current void onDone(@Nullable Throwable e, @Nullable T result) callback and inspecting all of its usages. The issue does not name files or tests; completion would require an agreed Result API design and migration of the affected callback surface, including its compatibility implications.

Written by the indexing model from the issue text.