fix: passing getUser exceptions to Method Channel

[luisfelipeas5]

Uses FlutterResult on SwiftRoamFlutterPlugin.swift (iOS) and Result on RoamFlutterPlugin.java to return for the flutter the errors of getUser method.

With this fix refactor, we can use try catch, like:

try {
  await Roam.getUser(
    userId: "",
    callBack: ({user}) => debugPrint(user),
  );
} catch (exception, stackTrace) {
  log(
    "Roam method invokation exception: "
    "methodName = getUser "
    "exception = ${exception.toString()}",
    stackTrace: stackTrace,
  );
}

Note for future improvement: could be better not have callbacks on this methods to allow this type of code:

final user = await Roam.getUser(userId: "");
debugPrint(user);