Using an optional return type and returning nil to signify a failure.

nil-as-error.swift

class AnObject {
  func resultOfWork() -> Any? {/* ... */}
}

func doSomeWork() -> Any? {
  guard let result = anObjectInstance.resultOfWork() else {
    // log and/or show error alert here?
    return nil
  }
  // ...
  return result
}

if let result = doSomeWork() {
  // work with result
} else {
  // or log and/or show error alert here?
}