Repeated cause getting in `ModuleSerializationProxy`

[djspiewak]

Specifically, note the following:

    override protected def computeValue(cls: Class[_]): Object =
      try getModule(cls)
      catch {
        case e: PrivilegedActionException =>
          rethrowRuntime(e.getCause)
      }

And then in rethrowRuntime:

  private def rethrowRuntime(e: Throwable): Object = {
    val cause = e.getCause
    cause match {
      case exception: RuntimeException => throw exception
      case _ => throw new RuntimeException(cause)
    }
  }

You'll note that we're doing getCause twice here. I'm not sure that this is actually correct, since it would seem to mask the inner exception. Poking @retronym for clarification. I can definitely confirm that, when this code path is hit, the exception it generates is very unhelpful and does not contain the initial cause.

[lrytz]

PR that added this code: scala/scala#7297

[lrytz]

Pretty sure it's "just" an oversight