This needs to throw an exception if v is false, as if [scala.concurrent.Promise.complete](https://github.com/scala/scala/blob/v2.10.3/src/library/scala/concurrent/Promise.scala#L54) had been called.

Actually I think it would be better (less code and more efficient) to put the Scala code I sent for completeWith and tryCompleteWith into FPromiseHelper and then call FPromiseHelper from the Java code.