[SPARK-2320] Reduce exception/code block font size in web ui

Author: Reynold Xin <rxin@apache.org>

Closes #1261 from rxin/ui-pre-size and squashes the following commits:

7ab1a69 [Reynold Xin] [SPARK-2320] Reduce exception/code block font size in web ui

core/src/main/resources/org/apache/spark/ui/static/webui.css

@@ -95,6 +95,10 @@ span.expand-details {
   float: right;
 }
 
+pre {
+  font-size: 0.8em;
+}
+
 .stage-details {
   max-height: 100px;
   overflow-y: auto;

core/src/main/scala/org/apache/spark/ui/jobs/StageTable.scala

@@ -168,7 +168,7 @@ private[ui] class FailedStageTable(
 
   override protected def stageRow(s: StageInfo): Seq[Node] = {
     val basicColumns = super.stageRow(s)
-    val failureReason = <td valign="middle">{s.failureReason.getOrElse("")}</td>
+    val failureReason = <td valign="middle"><pre>{s.failureReason.getOrElse("")}</pre></td>
     basicColumns ++ failureReason
   }
 }