[SPARK-19432][CORE] Fix an unexpected failure when connecting timeout

## What changes were proposed in this pull request?

When connecting timeout, `ask` may fail with a confusing message:

```
17/02/01 23:15:19 INFO Worker: Connecting to master ...
java.lang.IllegalArgumentException: requirement failed: TransportClient has not yet been set.
        at scala.Predef$.require(Predef.scala:224)
        at org.apache.spark.rpc.netty.RpcOutboxMessage.onTimeout(Outbox.scala:70)
        at org.apache.spark.rpc.netty.NettyRpcEnv$$anonfun$ask$1.applyOrElse(NettyRpcEnv.scala:232)
        at org.apache.spark.rpc.netty.NettyRpcEnv$$anonfun$ask$1.applyOrElse(NettyRpcEnv.scala:231)
        at scala.concurrent.Future$$anonfun$onFailure$1.apply(Future.scala:138)
        at scala.concurrent.Future$$anonfun$onFailure$1.apply(Future.scala:136)
        at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
```

It's better to provide a meaningful message.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16773 from zsxwing/connect-timeout.

core/src/main/scala/org/apache/spark/rpc/netty/Outbox.scala

@@ -56,7 +56,7 @@ private[netty] case class RpcOutboxMessage(
     content: ByteBuffer,
     _onFailure: (Throwable) => Unit,
     _onSuccess: (TransportClient, ByteBuffer) => Unit)
-  extends OutboxMessage with RpcResponseCallback {
+  extends OutboxMessage with RpcResponseCallback with Logging {
 
   private var client: TransportClient = _
   private var requestId: Long = _
@@ -67,8 +67,11 @@ private[netty] case class RpcOutboxMessage(
   }
 
   def onTimeout(): Unit = {
-    require(client != null, "TransportClient has not yet been set.")
-    client.removeRpcRequest(requestId)
+    if (client != null) {
+      client.removeRpcRequest(requestId)
+    } else {
+      logError("Ask timeout before connecting successfully")
+    }
   }
 
   override def onFailure(e: Throwable): Unit = {