FIX: rememberDuration reassignment error message

I was reading throught the scheduler and found this small mistake.

Author: Guillaume Poulin <guillaume@hopper.com>

Closes #8966 from gpoulin/remember_duration_typo.

streaming/src/main/scala/org/apache/spark/streaming/DStreamGraph.scala

@@ -38,9 +38,7 @@ final private[streaming] class DStreamGraph extends Serializable with Logging {
 
   def start(time: Time) {
     this.synchronized {
-      if (zeroTime != null) {
-        throw new Exception("DStream graph computation already started")
-      }
+      require(zeroTime == null, "DStream graph computation already started")
       zeroTime = time
       startTime = time
       outputStreams.foreach(_.initialize(zeroTime))
@@ -68,20 +66,16 @@ final private[streaming] class DStreamGraph extends Serializable with Logging {
 
   def setBatchDuration(duration: Duration) {
     this.synchronized {
-      if (batchDuration != null) {
-        throw new Exception("Batch duration already set as " + batchDuration +
-          ". cannot set it again.")
-      }
+      require(batchDuration == null,
+        s"Batch duration already set as $batchDuration. Cannot set it again.")
       batchDuration = duration
     }
   }
 
   def remember(duration: Duration) {
     this.synchronized {
-      if (rememberDuration != null) {
-        throw new Exception("Remember duration already set as " + batchDuration +
-          ". cannot set it again.")
-      }
+      require(rememberDuration == null,
+        s"Remember duration already set as $rememberDuration. Cannot set it again.")
       rememberDuration = duration
     }
   }