[SPARK-4204][Core][WebUI] Change Utils.exceptionString to contain the inner exceptions and make the error information in Web UI more friendly

This PR fixed `Utils.exceptionString` to output the full exception information. However, the stack trace may become very huge, so I also updated the Web UI to collapse the error information by default (display the first line and clicking `+detail` will display the full info).

Here are the screenshots:

Stages:
![stages](https://cloud.githubusercontent.com/assets/1000778/4882441/66d8cc68-6356-11e4-8346-6318677d9470.png)

Details for one stage:
![stage](https://cloud.githubusercontent.com/assets/1000778/4882513/1311043c-6357-11e4-8804-ca14240a9145.png)

The full information in the gray text field is: 
```Java
org.apache.spark.shuffle.FetchFailedException: Connection reset by peer
	at org.apache.spark.shuffle.hash.BlockStoreShuffleFetcher$.org$apache$spark$shuffle$hash$BlockStoreShuffleFetcher$$unpackBlock$1(BlockStoreShuffleFetcher.scala:67)
	at org.apache.spark.shuffle.hash.BlockStoreShuffleFetcher$$anonfun$3.apply(BlockStoreShuffleFetcher.scala:83)
	at org.apache.spark.shuffle.hash.BlockStoreShuffleFetcher$$anonfun$3.apply(BlockStoreShuffleFetcher.scala:83)
	at scala.collection.Iterator$$anon$13.hasNext(Iterator.scala:371)
	at org.apache.spark.util.CompletionIterator.hasNext(CompletionIterator.scala:30)
	at org.apache.spark.InterruptibleIterator.hasNext(InterruptibleIterator.scala:39)
	at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:327)
	at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:327)
	at org.apache.spark.util.collection.ExternalAppendOnlyMap.insertAll(ExternalAppendOnlyMap.scala:129)
	at org.apache.spark.rdd.CoGroupedRDD$$anonfun$compute$5.apply(CoGroupedRDD.scala:160)
	at org.apache.spark.rdd.CoGroupedRDD$$anonfun$compute$5.apply(CoGroupedRDD.scala:159)
	at scala.collection.TraversableLike$WithFilter$$anonfun$foreach$1.apply(TraversableLike.scala:772)
	at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:47)
	at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:771)
	at org.apache.spark.rdd.CoGroupedRDD.compute(CoGroupedRDD.scala:159)
	at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:263)
	at org.apache.spark.rdd.RDD.iterator(RDD.scala:230)
	at org.apache.spark.rdd.MappedValuesRDD.compute(MappedValuesRDD.scala:31)
	at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:263)
	at org.apache.spark.rdd.RDD.iterator(RDD.scala:230)
	at org.apache.spark.rdd.FlatMappedValuesRDD.compute(FlatMappedValuesRDD.scala:31)
	at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:263)
	at org.apache.spark.rdd.RDD.iterator(RDD.scala:230)
	at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:61)
	at org.apache.spark.scheduler.Task.run(Task.scala:56)
	at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:189)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:662)
Caused by: java.io.IOException: Connection reset by peer
	at sun.nio.ch.FileDispatcher.read0(Native Method)
	at sun.nio.ch.SocketDispatcher.read(SocketDispatcher.java:21)
	at sun.nio.ch.IOUtil.readIntoNativeBuffer(IOUtil.java:198)
	at sun.nio.ch.IOUtil.read(IOUtil.java:166)
	at sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:245)
	at io.netty.buffer.PooledUnsafeDirectByteBuf.setBytes(PooledUnsafeDirectByteBuf.java:311)
	at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:881)
	at io.netty.channel.socket.nio.NioSocketChannel.doReadBytes(NioSocketChannel.java:225)
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:119)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
	at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
	... 1 more
```

/cc @aarondav
Yes, the semantics we provide is that the HistoryServer or the Master can read event logs from previous Spark versions. We need to maintain backward compatibility for all classes used in these events to provide that guarantee.