some cleanup after Viktor’s comments

- remove useless `cause` argument from some akka exceptions
- improve names of some method arguments
- eliminate some closure allocation

akka-agent/src/main/scala/akka/agent/Agent.scala

@@ -214,7 +214,7 @@ class Agent[T](initialValue: T, system: ActorSystem) {
   /**
    * Resumes processing of `send` actions for the agent.
    */
-  def resume(): Unit = updater.resume(inResponseToFailure = null)
+  def resume(): Unit = updater.resume(causedByFailure = null)
 
   /**
    * Closes the agents and makes it eligible for garbage collection.